The ability to enter a percent is coming in the next iteration of the GRSS. It will take the total DKP recorded and deduct that amount from the current.
For example
Joeblow has 10 DKP
He gets an item and spends 50%, leaving him with 5 DKP Total.
He gets another item and spends 50% of that amount, leaving him with 2.5 DKP.
That's how it's going to work.
The item popups in the next version are going to wait until out of combat to prompt for point values. Certain items shouldn't prompt, like shards, and I'm looking into an option to exclude Gems as well.
That sounds exactly like what we need. You've done a great job with this website and add on. Thanks!!