a redstone threshold upgrade for the drawers . Maybe would need 3 of them set at 25% 50% 75%
So if you had a draw with the 75% threshold upgrade in once the drawer got above 75% it would give a redstone signal out

There is already a redstone upgrade which outputs a variable signal strength based on capacity
There are another two upgrades as well which will give a redstone signal for the most-filled slot, and the least-filled slot