Fill the case to the bottom of the bullet...not compressed. That is your max load, reduce it 30% and that's your starting load. According to Hodgdon you are safe at the max. I used this method for loading 45-90 and stuck with the max of 17grs.
FWIW I am loading 6.2gr of TB with a 200gr bullet in 44spl. Given the difference in case capacity between 44spl and 45 Colt I would say you would be safe upping my load by a grain for 200gr 45 bullets.
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