First two definitions:

1. Extended hit: An extended hit is a hit that comes on the third or fourth play of a doublet.
For example, if the target is 12 points away, 66 hits "naturally" because it can reach the target using just two of the four available 6s. 44 and 33 require the third and fourth play of the doublet, respectively, and therefore are extended hitters.

2. Two-folder: a pair of numbers such that one is twice the other.
Example: 63, 42, and 21 are two-folders.

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With that in mind, you can quickly derive the number of "hitters" as follows:

For direct shots (i.e. within 6 points of target):

1) Add 10 to the distance
2) Add 1 for each extended hitter

Example. You have a direct shot at a blot 6 points away.

1) 6 + 10 = 16
2) 16 + 1 = 17 (22 is an extended hitter at a distance of 6, coming on the third available 2)
3) 17 shots out of 36 = 47%

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For indirect shots:

1) Subtract the distance from 13
2) Add 1 for each extended hitter

Example: You have an indirect shot at a blot 9 points away.

1) 13 - 9 = 4
2) 4 + 1 = 5 (33 is an extended hitter)
3) 5 shots out of 36 = 14%

For double direct shots:

1) Add the distances together
2) Add 16
3) Add 1 for each uncounted extended hitter
4) Add 1 if the distances are a two-folder

Example 1: You have direct 6s and direct 3s to hit the target.

1) 6 + 3 = 9
2) 9 + 16 = 25
3) 16 + 2 = 27 (11 and 22 are extended hitters)
4) 27 + 1 = 28 (The 63 pair is a two-folder)
5) 28 shots out of 36 = 78%

Example 2: You have direct 4s and 2s to hit the target.

1) 4 + 2 = 6
2) 6 + 16 = 22
3) 22 + 0 = 22 (Note that 11 is an extended hitter for 4, but it shouldn't be added since it already hits "naturally" as a 2)
4) 22 + 1 = 23 (The pair 42 is a two-folder)
5) 23 out of 36 shots = 64%