A weight-tape is brilliant, though, for monitoring condition and seeing whether a horse is losing, maintaining or gaining. But for assessing whether a horse is overweight or not, condition scoring is better.
My boy is a 16.3 MW and I worked out his weight using the girth and length measurement calculation and he came out as 542Kg and a weight tape puts him at 545kg. I can easily feel his ribs and when he trots on the lunge for example you can just see them.