When we talk about prison reform, we talk about the three main functions of prison: incapacitation, rehabilitation and deterrence. Corporate persons are conceptual, amoral agents incapable of personal growth, so I don't think rehabilitation is a useful lens for thinking about corporate fines, but I think deterrence and incapacitation are worthwhile. Fines as a percentage of corporate income speaks to incapacitation -- a corporation with less money can do less harm -- but it doesn't address deterrence. In a perfectly efficient capitalist economy, we should expect that corporations will take any option that nets more revenue than it costs to implement, inclusive of any fines, and the fine is only deterrent if it makes the offense so expensive as to be unprofitable, which is only loosely correlated with corporate income. I'd say, it's the maximum of: a percentage of corporate income, the cost to fix the damage, and the amount of profit the corporation derived from the fined behavior.