A S.W.A.G. always worked well for me as I really don't think plumbers are very good with numbers, at least that is what I feel from the competitive bids faced with lately. I use to tell customers I have squares written on the floor at the shop with various numbers in them and from 20 feet I throw a coin, whatever square it falls into that is the price, I don't think they believed me so I told them it's called a S.W.A.G. or "scientific wild ass guess", but the only real way is to figure your time and materials, put together for your bid, with time & money you'll get better, rich or hungrier.