I used pre-seed on a couple of occasions ... when I purchased it in came in sachets, which said to only use once. However we found there was enough in the sachet for 3-4 uses, so throwing it out after one use would have been a total waste.
I used to peg the top of the sachet shut and store it in the fridge.
Having said that, I don't think you can use too much - use however much makes you feel comfortable. DTD shouldn't be a painful chore