One could always establish a minimum-living income that was immune to fines based on ratio of income, or consider specific expenditures to be protected from the calculation (e.g. rent or cost of travel to work). However, I can see how that could eventually lead to the truism becoming "If the penalty for a crime is an income-ratio fine, then that law only exists for the middle class, and not for the lower or upper class" which would be... interesting.