Your assumptions sound correct.
For forward planning I just assume what happened in previous years will continue to happen. It's not quite as simple as that, as GreatLaker's links detail, but for planning it's close enough.
Example:
Base Salary = $80,000
Overtime/Pager Pay = $20,000
Pension Adjustment = $11,000 (~13.75% Base Salary)
RRSP Room:
18% of $100K Gross = $18,000
Pension Adjustment = -$11,000
RRSP room for following year = $7,000