I don’t get your point. What would you achieve by basing the decay % on your total amount of ward?
With having ward decay being a fix percentage of your currently amount of ward the pure number of ward that decays per second is already increasing with your current ward.
20% of 100 ward is 5 ward decay
20% of 200 ward is 10 ward decay
So the higher your ward is the faster it decays.
Now if you would also increase the percentage of ward decay based on the current amount of ward the rate of decay would be increasing more with gaining more ward.