Ah, yes. Those good old "Black Art" days.
I prided myself being quite good at it when most aircraft didn't have DME and we had no radar. It was always satisfying to hear the acknowledging mic blips, right on the calculated time as the aircraft passed, just prior to the pilots reporting the visual passing. Unfortunately, I too have forgotten the formula, but it was based on the ETAs/ATAs at reporting points either side of the crossing point. Anybody got an early 1960's (green) 1st edition of the NZ MATS?