I fly too many 737 variants (both classics and NG's) at too many grossweights to remember all these rules of thumb. Just this week I have seen Vrefs as low as 120 kts and as high as 170 kts on two subsequent legs on the same day.
For me, 55% N1 usually works fine as an initial setting (60% for flaps 40, 65% for single engine/flaps 15). Of course it's a little higher if you're heavy and a little lower if you're light, but I like round numbers, and I rather start with a pretty rough but workable estimate than with an accurate "guesstimate" for the wrong subtype! And anyway, no matter how good you estimate the target N1, it still remains just that: a target value.