Basic guidelines:
Sit on it. Does it hurt?
Go ride. Does it hurt yet?
Wait a day. Does it hurt now?
A guy at NAHBS gave me the sit bone measurement and told me 135mm but I'm schlubby and that was in jeans... the Rivet saddle guy also gave me a good sell. But I like the saddles found on low-mid grade mountain bikes.
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Genesis 49:16-17