I imagine they will vary dependent on conformation of your horse - inevitably an accurate tape for a TB is likely to be less accurate used on a cob or native. Unless you're calculating doses, in which case a weigh bridge may be advisable, tapes are really only useful for keeping an eye on weight fluctuations rather than giving an accurate empirical value to your horses weight (which is largely irrelevant to condition).
Agree, only any use to monitor gain or loss, no use to obtain a figure. When mine was 500kg of muscle on a weighbridge, weigh tapes gave measurements between 400 & one of almost 600.