NFL officials have a bye week, so that just bolsters your percentage argument.
1/16 = 6.25%
However, officials' expected level of professionalism should be higher than players and coaches in my opinion.
Still, 6.25% is a considerable chunk, but it is 6.25% of a much smaller salary, the dollar amount is much smaller, and part-time officials are expected to have some other form of income.
I recall the NFL issuing fines based on dollars and not game checks most often. If fines were always issued on a game check or percentage basis, the deterrent would be less effective for the less compensated individuals.
If a player spits at an official
and an official returns fire
(I would hope not), how would we fine each? Even dollar figures? Even percentage? Progressive or regressive scale? I think the current fines are about right to provide the proper deterrent for the actions involved (taking into account individual histories).