What wheel backspace should I use on CPP Modular spindles

[Ontcalimo]

Hello, I would like to get advice from members that have CPP modular drop spindles on their 67-68 c10's with CPP disc brake system. I like to run a 20 X 8.5 wheels on the front. It has a static drop of about 4 inches in the front. I am not sure what backspace I used use. I like to purchase a set of wheels which I will likely run a 45 series tires, like a 245 or 255X 45 20's. I do not want wheels rubbing stock inner fenders. There are few posts that talk about this subject, but I noticed many members who do not have this system give their opinions. Their recommended measurements vary from 4.5 to 5.75. and this confuses me. I like to hear from members that actually have this system or have accurate prior knowledge about this subject. My understanding the CPP modular spindles bring the wheels about 3/4 (.75) inches narrower (however some dispute these figures). I'd like to hear from them what works measurements works and what backspacing will not rub fenders. I also like to hear what backspace they have used on rear wheels on 20X10 with early rear end with a drop of at least 6 inches in the rear.