I have used the same calculator of FM12 to calculate the stars of the coaches.
It's all ok except that for the GOALKEEPING COACHES where the stars calculated doesn't correspond to those of the game.
In the calculator the attributes for GOALKEEPING COACHES are: Goalkeepers - Determination - Level of Discipline - Motivating.

If you want to tray use these coaches:
Bobby Mimms: 18-14-13-16 (with calculator 4 stars)
Joe Corrigan: 19-15-15-15 (with calculator 4,5 stars)

According to the calculator Corrigan is better than Mimms (it's clear also at first glance).
When I flag Stop Shooting Mimms has 4 stars (it's ok), Corrigan has 4 stars (not 4,5).
When I flag Handling Mimms has 4,5 stars (not 4), Corrigan has 4 stars (not 4,5).