When calculating strike percentage, divide total strikes by total number of frames. In the case of strikes it would be 12 frames because a game can yield 12 possible strikes.

When calculating spare percentage, divide total spares by total number of frames minus strikes. In the case of spares it would be 10 frames because a game can yield 10 possible spares.

If you want to get real technical, you can factor in close to impossible splits. In that case you would subtract the number of frames that yielded close to impossible splits from the number of total frames. In addition, of course, you would subtract the number of strikes thrown.

That is the way I like to do. Another way to consider is, disecting the 10th frame for each game. If you throw a strike and a spare in the 10th, you would still calculate spare percentage based on 10 potentional spares. If you throw 2 strikes and a 9 or whatever it may be in the 10th, you would calculate the game based on 9 frames where possible spares may occur.

Strike percentage needs to take into account the number of times you threw at a full rack.  For some people that may be 30 times and some could be 36 and even others may be anywhere in between.

For example, last night I punched out in the 10th of the 1st game, so that would be 12 strike attempts.  Game 2 was a double and count, so again, 12 attempts.  In the last game I went spare-strike, so there were only 11 attempts.  For the three games, it was a total of 35 shots at the full rack.

I found a great program on the web that allows you to track every ball you threw..  I can tell you that I am currently shooting 77% of my single pin spares..  90% of my two pin spares...  65% of 3 count spares.. and so on..  I can tell you my split conversion sucks.. lol..  and that I am shooting strikes about 34% of the time..  All on a PBA pattern shot...  Let me know if you want it..