I'm a homeowner, tackling an above-average SRW project (in terms of sheer size). I know it's going to take me all summer and many beer/food bribes to get my friends over here to help. Anyhow... I have detailed plans drawn out (my wife is an architect) and know precisely how much square footage/cubic yards etc, I need. My question, though, is how much extra should I plan for? I know when, for example, laying tile, you account for 20-25% extra for cuts/mistakes, etc. Would buying 20% extra materials (blocks/gravel) be enough? I would like to avoid last minute delivery expenses/delays, but of course don't want to be stuck with a bunch of materials left over. Thx.