10% hurts the same no matter your income.
Fines are about punishment and deterrence. You cant deter a millionaire with a 100$ fine like you can a pensioner on a fixed 1000$ income.
No, 10% of income doesn't hurt the same no matter your income. (Even if you ignore the relationship loose correlations between income and savings that can be used to cushion the effects of unexpected expenses, and assume neither party has any such resources.)
While fungibility pushes slows the decline compared to less-fungible goods, declining marginal utility applies to income, too, which means not only does a flat fine have less impact on the rich, so does a flat percentage.
(This gets even more true when you do consider savings, etc.)
Even better might be a percentage of disposable income but even that is not going to be enough and with more complexity in the rules comes more opportunity for creative accounting which again benefits those more well off.
Id rather have it scale badly than not at all though.
10% is more just. Than a flat fine.
But 10% hurts more when you are poor and have no savings and you need that money to pay rent. For small companies is the same. Bigger corporations have more resources available to minimize that 10% impact. Power does not scale linearly with money.
But to take a percentage is a much better way than a flat fine. Flat fines are totally ineffective when applied to big corporations.