Orginal Rhoads Lifters reduce lift and duration at idle by approximately .010" to .020". Duration is reduced by approximately 10 to 15 degrees. Total lift and duration are restored at approximately 3500 rpm. It's not uncommon to pick up 2-3" of vacuum under 1500 RPM just by using these lifters. You will normally also see an increase in low end torque.
Oil viscosity, valve spring pressure and engine temperature all combine to affect the total leak-down of the lifter at idle. Therefore total leak-down is only approximate.
611-9008 Instructions (PDF)
351C, 351M, 400M