There is no easy way around this. Even AppStore app FormatNumber does not increase the max number of 8 digits.
Your best option is to store your attribute in a string and use AppStore module Math for your calculations (if any). See https://mathenginetestapp-sandbox.mxapps.io/p/simple, add “0.12345678 / 100” or "1.2345678901234 + 1.2345678901234” to the formula, to see it at work.