I actually think the weight tape is generally pretty accurate, and suspect that it is often 'pilot error' when it is wildly off(as in Charlotte's example.)
It does need to be used EXACTLY according to directions.
I use a weight tape everytime I worm or medicate anything according to weight; also give my vet my weight tape amounts when he needs them for medication-have never had any issues.
My medium build, 24 YO, 34" mare tapes at around 260 lbs., has been as high as 275 when she was hefty. both of my 34", medium build geldings(one, the son of the mare above, and a bit finer in build than she), tape at 235-260, depending on condition; his 33.5" full brother,finer built,tapes at around 220-235, again, depending on condition and whether he's 'fit' or somewhat 'fat'! My substantial, but not drafty, 38" mare tapes at around 375-380 lbs.,while my 30.25 mare, around 180 175-180 lbs. Both 32" horses are around 210-220 lbs.(the gelding is a heavier build, and is usually the heavier of the two.) Reading through this thread, it seems that these weights are well in line with most of those of comparable height/build, listed here, which have actually been weighed.
I have a Rhodesian Ridgeback female who weighs 130 lbs. on the vet office scale-and though quite large for a Ridgeback, she is clearly 'WAY lighter than even my smallest(the 30.25") mini-another reason I feel pretty comfortable about my weight-tape weights being pretty accurate.
Margo