tjs: I believe that a break in comunication has occured. A perfect possible score for 30 targets is 330. You can look at that in any way you wish. 30 - 11 pt targets or 30 - 10 point targets with an additional 1 point for every X. 30 x 10=300 + 30 possible points for X' s = 330. No matter which way you view it the total possible is 330 pts.

Using your example of scoring hits it would work like this:
(9 X' s) 9x11 = 99
(20 - 10' s) 20x10 = 200
(1 - 8) 1x8 = 8
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99+200+8 = 307

If you wish to subtract:
(9 - X' s) 9x0 = 0 (330 - 0 = 330)
(20 10' s) 20x1 = 20 (330 - 20 = 310)
(1 - 8) 1x3 = 3 (310 - 3 = 307)

I think what is throwing you is that you are failing to subtract the appropriate number. The appropriate scoring now is :

X' s = either +11 or -0
10' s = either +10 or -1
8' s = either +8 or -3
5' s = either +5 or -6
misses = either +0 or -11

I promise you that this is correct. Hope it helps.