We just do full first level hp, then half plus one for each additional level (d4=3, d6=4, d8=5, d10=6, d12=7).
We had a couple of suspisciously high hit point totals, one of which came from a rather tall & burly guy (very intimidating guy). Not wanting to point fingers, we just quickly settled on this method as a fair way to make sure cheating stayed out of the game. Same reason we switched to point buy.