I'm doing some work on streamlining my process for designing new aircraft, and in the process creating/looking for rules of thumb and shortcuts one can use when designing. I'm currently in the process of creating a way of quickly estimating L/D of an aircraft in conceptual design stage. I'm looking for something that doesn't involve a ton of CFD or windtunnel time, and can be incorporated into a spreadsheet. It needs to be +/- 10% of the actual L/D achieved.
I've been able to figure out lift and drag of individual components pretty easily. However, I'm having trouble with figuring a quick way of estimating interference drag. What shortcuts do experienced designers use for estimating interference drag quickly?