I don't hold it against anyone who wants to use a flat percent across the board, but I've found the sliding scale to be most useful. If I sell a 25 cent piece of hardware, it's going to bill out at a buck fifty. If I sell a 2-dollar receptacle, it will bill out at 10 bucks.
Works for great items that I only use a few feet of. If I pay 100 bucks for 1000' of cat5e, and I use the whole box on a job, I bill it out for $225. On the other hand, if I carry that box around for months, selling 50 foot drops here and there, it bills out at ($5 true cost x 4.25) $21.25 per drop. Selling it that way, I get $425 off that box of cable selling it piecemeal, versus $225 selling the whole box outright. Gotta get paid for hauling this crap around every day.
Works for stuff like connectors too. If I use a whole box of connectors on a job, I make one price. If I sell them a few at a time, over many jobs, I make even more.