Yeah, ~5% is what I use and it's never failed me, but FWIW, Richard Small concluded that 4.5% +/-10% or a 4.9% upper limit was a good rule-of-thumb. Oh well, what's 0.1% among friends?

Since we're more likely to hear sharp edge diffraction noise at these low vent machs, a slight rounding over is more important than whether a stock pipe size bumps it up to 6%.
GM