There's a few similar threads around. What you want to do is calculate the
net present value of the income stream. Since it's not linked to inflation, you're going to have to adjust each payment down by the expected value of inflation, so you need an estimate (3%/year is typical). You're also going to need to know your
life expectancy, as that determines how many payments you expect to receive. After you have those details, it's a geometric sum:
NPV = (ps)(1 + i)
a - r(1 - (1 + i)
r - e - 1)/i
Where
p = 0.15 (your percentage of final salary)
s = ? (your final salary)
i = 0.03 (expected inflation rate)
r = 65 (the age you retire)
a = ? (your current age)
e = ? (your life expectancy)
The 4% rule is inapplicable here because that calculates an inflation-adjusted income stream, which doesn't necessarily end when you die.