Problem with Trapping.

I have an equipment "i1Pro 2" that it can obtain a patch's C,M,Y and K densities.
I want to calculate Ink Trapping, but I encounter a problem in overprint patch.

EX: Preucil formula
patch1: solid cyan: (C:100, M:0, Y:0, K:0) C density is D1?
patch2: solid magenta: (C:0, M:100, Y:0, K:0) M density is D2?
patch12: solid blue: (C:100, M:100, Y:0, K:0) which one(C, M , Y, K) density for blue(overprint) D12?

Tr = ((D12 - D1) / D2) x 100
q1)is it C density for D1 in solid cyan patch?
q2)is it M density for D2 in solid magenta patch?
q3)in blue(overprint) patch. which one(C, M , Y, K) density is for D12?
q4)How do I minus paper?

thank for helping me.