I read about issues with the Service Canada online calculator this month in "MoneySense" magazine. It works if your income if you average income is unchanged up until until you start drawing CPP but if you retire early that won't be you. They recommend <
www.drpensions.ca> "a CPP expert can give you a more accurate projection of your future benefits" (fees start at $25). I imagine if you stop working at 51 it's in your best interest to start your benefits as early as possible at 60 (you can't collect before age 60 unless you are disabled. Old Age Security starts at 65, gradually increasing to 67 for those born in 1958 and later).
All the best Fortuna - hope this info is useful for you!