This is a common question that has been discussed at length here and elsewhere. Rather than trying to assign a value to the pension, make a conservative estimate of the annuity at the time you will take it. That income stream will not have to be generated by your assets. For example, if you were planning to retire at 60 and would receive a pension of $1,000 a month then, but anticipate needing $4,000 a month, your assets need to generate $3,000 a month. Using your desired withdrawal rate, work backwards to the amount of income-generating assets you will need then.
If you are planning to retire earlier, your assets will need to generate whatever income you need until the pension kicks in. If the pension is reduced, the lesser amount is plugged in. There are retirement calculators out there, such as firecalc and c-firesim that will crunch the numbers for you, including different incomes and start dates.