If the amount shown at line 308 is different than what is shown in box 16 of your T4, then likely the amount on the T4 is higher. Likely, the amount withheld by your employer for CPP contributions was greater than required...and the balance will show at line 448 of the T1 as a refundable credit.
Follow-up question... my line 308 does not reflect what is on line 16 of my T4 (the amount in TurboTax is lower by about $90). There is also an amount of $0.00 showing on line 448. Why are these numbers different in my case?