Starts missed = 1 / ((62.53/RA) + .0064). R is the injury Rating. A is the number of PAs an ideal player "would" have if he played every inning of every game. (See below.)
A big problem with figuring out each injury rating's "average starts missed per season" is that a full season of PAs varies based on two things: 1) Where the player bats in the line-up; and 2) whether the team is low-scoring or high-scoring. A guy who leads off every game on a very high-scoring team will have a lot more PAs than the #8 or #9 hitter on a low-scoring team. This difference in total full-time potential PAs affects how many starts are missed due to injury (especially for higher injury ratings).
REQUEST FOR DATA: If anybody has had a guy play (almost) every inning of (almost) every game at lead-off on a high-scoring team, I'd be interested in hearing how many PAs he had. Ditto for a guy who batted last in (almost) every game for a low-scoring team.
A while back I posted a complete formula for figuring how many starts would be missed for any injury rating (1 through 6), on average over a full season. The formula allowed for lots of different presumptions to be changed, including the PA question.
Here is a simplified version with some presumptions "built in" (rather than allowing for different presumptions), but still allowing for different PA totals. It's only marginally less accurate.
For a player with fewer than 600 AB+W on his card:
Starts missed = 1 / ((62.53/RA) + .0064). That is, the reciprocal of ((62.53/RA) + .0064). R is the injury Rating. A is the number of PAs an ideal player "would" have if he played every inning of every game.
To account for the "remainder of game" tacked onto each injury (to give the total for "games missed" rather than just "starts missed"), multiply the result by 1.136 (i.e., 13.6% more).
Here's an example of how much difference is made by the range of different "maximum PA" totals:
600: If an ideal every-inning player "would" have 600 PAs in 162 games (e.g., a #9 hitter on a low-scoring team):
injury 1 = 9.00 starts missed
2 = 17.05
3 = 24.29
4 = 30.85
5 = 36.81
6 = 42.25
(multiply all by 1.136 to get total games missed)
800: If an ideal every-inning player "would" have 800 PAs in 162 games (e.g., a lead-off hitter on a high-scoring team):
injury 1 = 11.75 starts missed
2 = 21.91
3 = 30.79
4 = 38.61
5 = 45.54
6 = 51.74
(multiply all by 1.136 to get total games missed)
You can see, it makes a big difference.
I don't have any idea how many PAs would be correct in ANY case, but the above formula allows the number to be adjusted.
(Note: the numbers above are based on a slightly more exact version of the formula. Also, PAs include every plate appearance except sac bunts and hit-and-runs, on which no injury is possible.)
Again:
Starts missed = 1 / ((62.53/RA) + .0064).
Games missed = 13.6% more than starts missed.