My uncle's "always round up" rule cost me $2.40 at the hardware store, and I'll defend it anyway
Everyone says his 1998 trick of rounding every board foot up to the next whole number is dumb, but when the lumber yard in Tacoma shorted me on a 6 foot piece, that extra quarter inch was the only reason my shelf still fit, so what's your go-to dumb math habit that keeps saving you?