You need to be -really, really careful with this- because IMO it's dodgy. Definitely worth a no-prop-test.
What I do is take the values from blheli configurator for min and max and divide by 4, and set those to min and max. Set neutral to exactly in the middle. Set channeldeadband to whatever is necessary to get props to consistently spin when armed.
Reverse a channel's min and max if necessary to get it to spin the right way.