Depends on the bike's suspension geometry & leverage on the shock too. Equal pressure to weight (in Lbs) is a starting point, but some will work better at around 90% to get the sag right, others may be 110%. Typically longer travel exerts more leverage, so needs more pressure. As JTMofo said, ultimately the correct sag is what you're after, so just play around with the pressure until you get it right.