Using Date Differences in Math Formula for Averaging

I have employees who are enrolled in performance plans throughout the year. Based on several factors, a participant may be assigned a different percentage of earnings during that time. I need to be able to average the different percentages to obtain the overall percentage the participant will be credited with for the entire year. I've attached an example of the various data scenarios I can encounter with explanations of what I'm seeking. A simplified example:

Todd Plan 1 1/1/2012 5%
Todd Plan 1 4/1/2012 10%

Todd is in "Plan 1" throughout the year. He is credited with 5% of earnings for 3 months (January through March) and 10% for the remaining 9 months (April through December). The formula used to obtain the average in this case would be

Formula:

 (3/12) * .05 + (9/12) * .10 

I can muddle my way to this point. It's when I deal with other scenarios that I encounter problems in how to use the power of Excel for solving. For instance, partial credit (when a participant is in the plan from the 16th of the month) is not given. If the particpant is in the plan from the 15th or earlier, then a full month is credited. In the example shown here, the participant does not get April at the 10% rate, but April is assigned at the 5% rate. So, she's credited with 5% of earnings for four months (January through April) and 10% for the remaining 8 months.

Mary Plan 1 1/1/2012 5%
Mary Plan 1 4/16/2012 10%

Formula:

(4/12) * .05  +  (8/12) * .10

Still, not too harsh. Other examples provided on the attachment reveal the true nature of what I'm attempting to resolve. Appreciate any guidance.