Hi Nancy
I have found that measuring girth plus length and calculating gives me the best results, as confirmed on weighing machine. However, you need to measure at least three times to make sure you’re accurate, and unless you have a small, non-wriggly horse, you need another pair of hands for the length measuring.
Ideal weight is what your horse weighs when they have a body condition score of 4.5. There’s a section explaining this in the Wiki, with photos. 

It took years for me to realise how low my horse’s ideal weight was, because I’d never seen her at a BCS of less than 6! 
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Maxine and Indy (PPID) and Dangles (PPID)

Canberra, Australia 2010
ECIR Primary Response

https://ecir.groups.io/g/CaseHistory/files/Maxine%20and%20Indy%20and%20Dangles 
https://ecir.groups.io/g/CaseHistory/album?id=933