"Wait that's it!" Harry exclaimed to himself as his eyes snapped open in his bed. If he didn't have a way to measure his mana he could just make one! All he had to do was decide on a basic unit of measurement and count how many he had, but what to use as the basis?
"It needs to be simple and something standard that can be replicated by anyone to keep all measurements accurate." Harry mumbled to himself as he started pacing back and forth. An idea came to the ten year old when he spotted his mother's Charms book laying on top of the cardboard box he kept all of them in. "That would work!" He nodded to himself as he sat back down on his bed and held his hand in front of himself. With barely a thought a small ball of light appeared above his hand, Harry's Wandless version of the Lumos Charm. "Now I just need to see how little mana can actually produce the spell." With that Harry set about withdrawing mana from the spell. The bright light shrank and dimmed a little each time Harry reduced the mana he used to conjure it. When Harry barely felt any mana flowing to the light it suddenly went out. He tried to reach the amount just before it went out a few times before getting it right. In Harry's hand was a small light no bigger than a pea that barely illuminated his palm.
"This is it, the smallest amount of mana that can actually make a Lumos Charm. Since the Lumos Charm is the most basic of basic spells it should be perfect to use to measure my mana." Harry muttered to himself with a grin. 'But how do I count it?' he wondered to himself mentally. 'I can only keep a few Lumos Charms going at once. But I feel like I could make a hundred of these things. I need something else than can count for me.' Harry mulled over his problem until he went to bed that night.
Harry still hadn't found an answer to his question by the time spring came around. He'd looked through his mother's books and notes and tried to make as many of the pea-sized Lumos Charms as possible only to falter when he got past ten at a time. In frustration Harry turned his last hope onto the fantasy books in his school library. Over a month later he'd stumbled upon an idea in one of the fantasy books. It had a concept called spell-forging in the story that was close enough to enchanting that Harry thought it could work. Spell-forging, according to the fantasy novel, was a method to make magical objects by imbuing the item in question with magic throughout its creation. Runes were then used to add an effect and lock the item into fulfilling that singular purpose. Harry was getting excited this mundane fantasy novel might very well have solved his problem! He only needed to find something he could make from scratch that he could also carve the proper runic sequence into.
Thus Harry's search began to find something he could make himself that was also big enough to carve or etch runes into. In a humorous twist Harry's least favorite craft from his summer day camp turned out to be his solution. Harry stumbled onto the idea to use clay for his project when they'd started using it for Art class. With his choice made Harry had asked Uncle Vernon for some clay to play with one day. Vernon hadn't batted an eye at the request, knowing his nephew's propensity for arts and crafts. Harry had his clay the next weekend and set about pushing his mana into it while shaping it into a square frame. Etching the runes into it after it had partially dried was taxing as he was using a box cutter razor blade he'd 'borrowed' from the school janitor's cleaning cart. The razor's thin edge was perfect for keeping the rune lines straight but dangerous to handle without the actual plastic housing to protect his fingers. Thankfully he'd finished without cutting himself and just had to wait a few days for his square to dry.
"Alright, time to try this out." Harry grinned as he picked up the runic clay square frame. While holding it in both hands Harry started feeding his mana into his creation. Harry was thrilled when he saw a number '1' made of light appear inside the frame. He'd only given the frame enough mana to produce the pea-sized Lumos Charm and it had read the amount as one unit. That meant that his runic sequence had been correctly etched and his Arithmancy calculations were correct. With a smile Harry proceeded to push more of his mana into the clay frame. The ten year old boy watched giddily as the number continued to climb; '10', '25', '40', '60', '90', '120', '150', '180', '210', '240', before it came to a stop on '270'.
"Two hundred seventy mana, huh, I wonder if that's a lot or not?" Harry pondered aloud before feeling his clay frame crumble apart in his hands. "What? Why? What's going on?" He fretted as his creation broke into dozens of small pieces. As he carefully picked up the pieces from the floor he noticed on some of the larger ones that the runes etched into them looked burnt and misshapen. "Was it not able to handle the mana? It was a first attempt, I should probably be happy it worked at all. It could have been worse, at least it didn't explode." Harry sighed as he continued to pick up the pieces and place them in the waste bin in the corner of his room. He'd have to take it out later since emptying their own bedroom waste bins was a chore both he and Dudley had to do. Still he'd done it and now he knew that his Arithmancy was correct and that his runic sequence would properly measure mana based on the unit of measurement he'd established. Maybe he could tweak the runic formula so that he didn't have to be holding the device to get a reading? He'd have to look into that, if he could somehow work his theoretical updated rune sequence into his glasses he'd be able to scan the people around him for magic. He might be able to find other Wizards and Witches! Harry sighed as he tried to calm down this would probably take a while to work out and he had to head down for dinner soon.