OK,
Resurrecting my ME degree...

Here is an equation to add the correct amount of oil to the fuel you already have to get the desired ratio of oil in the fuel. The definitions are as follows:

P1 (The original Percentage of oil estimated in the fuel)
P2 (The desired percentage of oil in the fuel after mixing)
V1 (The original volume of fuel you are starting with)
V2 (the volume of oil you need to add to the fuel to get desired percentage (P2))

V2 = ((P2 - P1) x V1) / (1 - P2)

If you add V2 to V1 and then divide it by the volume of the Nitro originally in the fuel(V3) you can find the new % of Nitro (P3)

V3 = .2 * V1 > .2 = 20%
P3 = V3 / (V2 + V1)

Gotta rest the hamster now.
Curtis