my friend used a weight tape for her cob,he was 100 stone.when he had to go for a op at the vets he was weighted properly and came up at 101 stone so very close i think.
I've found that if you measure vertically at the point where the girth goes, they're accurate. When Cat as ill, he was weighed at Newmarket 4 or 5 times, & each time the weigh tape was thereabouts.