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I've built up one rear wheel and yes you unironically need to measure everything by hand yourself unless they give you pic rel which of course based CHYNAman does but many brands don't and you also already need to know which spoke pattern you're going to lace the wheel with since that affects effective spoke length. 
Also you can't always trust the rim diameter number that the rim manufacturer gives you which sounds bizarre because why wouldn't they know that but sometimes its a few mm off and that can also fuck you