Dallas here. I will be glad to help you with this.
You are right. Shopify doesn't allow decimals to be put in the quantity bar. The reason for this is that when a quantity of a physical product like a t-shirt is selected there can't be 1.25 or 8.65432 of them. I do, however, see how this can be an issue for what you are doing.
In that case, I suggest putting your quantity numbers in a smaller format. If you put them in Centimeters then you will always just need to move the decimal over two places. This then wouldn't require you to any extra and unnecessary mental math.
You could also always put it in a completely different format too like this example:
4.5 meters = 45 units / 25.3 meters = 253 units. With this one, however, I can see it getting confusing for your customers so I suggest the first option.
I know this isn't exactly what you were hoping for, but I do hope that it helps! If you have any other questions please do let me know!