I think I agree with your preliminary proposal except it doesn't say anything about the amount of time each parent takes care of the child. For example, an 80/20 split with 50/50 costs would be just as unfair to the CP as the current system is for NCPs. And sometimes 50/50 parenting time isn't possible.
Also, the estimated cost of raising our two children was WAY below the actual cost. I live in a shared income state and our CS is 70/30 (me 30%) of the estimated cost. However, I did a spreadsheet with actual cost of what I spend with having the kids 90% of the time (mostly due to their dad's work schedule and his decision to move away) as opposed to 50% or less and I found that I, the CP, cover about 70% and the NCP covers about 30% even though his income is much higher than mine.
I have no problem at all with being held accountable for where the CS goes but I think that's a can of worms my ex doesn't want to open because it might mean an increase in his CS.
I'm not trying to argue or anything. Just throwing out some things you might not have considered.