Just to add a little to the above, here's the 'compression' formula that would apply:
For SSA's below 50.3289725:
rating_points_per_throw = -0.487095 * SSA + 34.5734
So, if the SSA after field A plays is determined to be 50, the rating_points_per_throw increment would be 10.21865. So a 52 would be rated at 979.5627 (rounded to 980). So your field B would be vastly more likely to average ~55 than 52. (actually 54.89301424356).