Skater
Rights accrued upto the changeover date are definitely protected, so you will have earned your IPP rights in '14 and if you retire anytime after that you would get your gratuity and IP, thus your deduction seems perfectly accurate to me. The further benefit post '15 will just not be as beneficial as it would have been if you hit that 21 years before '15.
I am in a similar position to you, with an IPP next summer and serving to 55 with a deferred option to leave in summer '15. I am still confident that the changes will only affect further pension increases post '15. This is going to sting those who earn their IPP rights after '15, but I think it should be minimal to our situation, unless of course you get promoted!!