There are probably many ways to estimate this. Here's one:
1) Find the CAGR that just allows a 30 year retirement with a 4% WR to succeed. With expenses taken at the beginning of the year and annual compounding of the resulting balance, that's ~1.31%.
2) Using that 1.31% return, pick a WR (e.g., 3%) and thus an annual spending (for 3%, $45K).
3) Calculate the balance remaining after 20 years. For 3%, that is $911,277.
4) Subtract the $15K/yr pension from the annual spending and calculate the new WR. For the numbers above, we get ($45,000 - $15,000) / $911,277 = 3.3%.
5) Compare the new WR to the 4% Safe Withdrawal Rate expected to allow a 30 year retirement.
6) Adjust the WR in step 2 and iterate until the WR calculated in step 4 is 4%.
An initial WR of 3.224% meets the conditions specified above.
Of course, different assumptions are likely to yield different answers. Good luck!