To answer the question, 'I wonder how they figure what the interest rate drop is among the 3 used when calculating benefits? " The actual breakdown is in the pension documents but to simplify, back in 2009 they changed the interest rate used from 30 year US treasury bond rate (currently 1.41%) to a mix of short, mid and long term (investment grade) corporate bonds. I think it's split 1/3 to each segment but I'm not 100% sure. It's crazy but it's based on the rate at the end of the month, in our case it's the August month end rate which is then used for all lump sum calculations for the following year.
Even thought the pension is frozen, it does sort of grow. Every year you age your penalty for early withdrawal is reduced. Age 63 -65 it's 8%. Let's say you are 63 and you decide you want to take the lump sum based on the Aug 2020 rates. You don't have to take it in Jan. Let's say it's 300,000 if you take it in Jan 2021. If you wait until Dec 2021 the early withdrawal penalty will be reduced and you lump sum should be ~323,000. Now here's the important part. Let's say interest rates Aug 2021 are up an average of .5%. If you wait to take the lump sum until Jan 2022 you'll receive about $307,000.
Run your numbers! Understand the important dates. It's your money!