Wastage on my boats is around 2%
You can call the metal suppliers with your materials lists and get an estimate. I supply all materials lists with the plans. Ask if the price includes shipping, from some shops it does, from some it doesn't. It adds up to quite an expense some times. Some times one supplier will give you the best price , somtimes another will, so try them all, every time . They often promise short delivery times just to get the order. Then after they have your mony, they take their time. Make sure you state " NO SUBSTITUTIONS", or they will throw in odd ball sizes that will make the job far more difficult. Make sure they use zinc rich epoxy primer. If they use international Nu Plate F or some vinyle based primer , you will have to blast it off , nullifying the advantage of shotblasted and pre-primed steel.
Brent