Hi Minerva
Sorry its taken so long to get back to you

I have found the information it involves some thing called the pearson square.

(all in % alcohol by volume)
Here is the pearson square,
A D
C
B E
A = alcohol content of spirit to be added
B = present alcohol content of wine
C = desired alcohol content
D = diffrence between B and C
E = diffrence between C and A
The proportion D to E is the proportion of spirit to wine to achieve the desired strength.
If you are blending two wines of known strength and wish to know the final strength, the formular is:
(A X B) + (C X D) divided by A + C
A = No. of parts of 1st wine
B = Strength of 1st wine
C = No. of parts of 2nd wine
D = Strength of 2nd wine
for example
(2 X 15) + (3 X 10) Div by 2 + 3 = 60 div 5 = 12%
From First steps in wine making by C.J.J.Berry
Hope this helps and not confuses

Steve C