Help with Ride height on bags

[Henner]

Bare with me here while I explain the best I can. PO installed bags on 68 short box, they did not run any front bag cups, they just sit the bag in lower A arm. Truck previously had 15" wheels on it. Bags just have Schrader valves right now. The ride height i liked was front fender 28" from ground and rear 29" from ground. I now have installed 2" lowering blocks in the rear and installed 20's all the way around. Well when installing the 20's of course it raised the truck up all the way around so i let out some pressure to where the the fenders were just level with rubber on tires (no gap in fender wells). Took it for a drive and had to turn around and come back because i could tell if i ran over a quarter if it was heads or tails the front was bottoming out so bad. Then realized for the truck to be as low as i liked with the 20's installed there was very little air in the front bags. I have purchased New D2600 bags, CPP drop shocks, and CPP 2 1/2" lowering spindels. Once i install the front bag cups which are 2" then install Spindels am i technically only gaining 1/2" of drop?? I know many guys on here are running 20's on bags and driving around much lower than i would like to be at. Thank you in advance for any help on this subject. I'm learning as i go and this forum has been a huge help!!!