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Assuming the smoker will be sitting level how much is needed, both towards the center of the smoker and the grease drain? Will have a piece of angle iron in the center of the smoker running to a piece of 1.5" pipe as a drain....
If it's only for drainage and for cleaning discharge, any pitch will give liquid the necessary incentive to go downhill. I you have an RF, and are concerned about proper smoke and heat movement, you should probably contact the builder for his recommendation. If you built an RF yourself, you may want to ask some of the more experienced RF users. I am using a straight offset and don't have any experience with RF. That's my $0.02 worth. Good luck, Joe.