It's difficult to say as factors like slope of taxiway make so much difference and you really cannot assess how much upslope or downslope you are on unless it is significant. All I can say is on an RB211 powered 747, at heavy weights (340 tonnes +), anything up to 1.09 EPR to be sure of getting moving. Above about 280 tonnes, anything above 1.06 should get you going depending upon slope. There is no usable formula because you are affected so much by wind, slope, sticky brakes.