Best way to store percentages in a smart contract

Question

I need to store the IRR (internal rate of return) of a project in my smart contract.

Since Solidity can't store decimals it must be saved as an integer.

Which of these options would be better and why:

1. Save only the return (in tokens) and calculate the rate/percentage outside the contract.
2. Represent the percentage in the same way decimals on tokens are stored, i.e. by multiplying by 10 ** decimals().

Or is there a better way?

Answer 1

Just use fixed point maths, so option 2. Just use it to represent values between 0 and 1 with the accuracy you need (it's much easier than working with actual percentage). To get percentage, just pultiply that number by 100.