Well right now, in the prototype, the thresholds are just values. So like the low threshold is 10 protein and if you have less then it will try to make more. So yeah you're right they could be set as percentages of a storage value, that would work fine as well.
I was thinking a little about the dynamics caused by changing the thresholds. Basically it's about tolerance. In this notation it's [low threshold, high threshold, vent threshold]. So if you have
[1,10,11] what that means is there is a large tolerance of values. If the value is 4 then the cell will do nothing it will happily swim around. Moreover it will quite often hit it's vent threshold because it might be on 9 and then make a kill and get a big boost and not be able to take it all. Conversely
[4,5,12] means the cell will actively balance the amount of that compound pretty much all the time. It will never be happy until the amount is in the [4,5] range. This means the cell will burn ATP somewhat unnecessarily (if the processes that affect this value need ATP). Like if the value is oscillating between 3 and 7 in the thresholds above will be happy and do nothing whereas these thresholds will cause the cell to keep producing / using up the compound while that oscillation continues.
Anyway, to get to your original question, what does more storage mean? If you are on [4,5,10] do you go to [4,8,13] which gives you a larger tolerance or do you go to [4,5,13] which means you maintain the same level but don't vent so frequently.
It's all quite interesting. Basically the amount of tolerance really equates to homeostasis. It's what range you are willing to live in. There are advantages to both being tolerant and being intolerant.