If 100X fails, try RR 74-307 rule but keep in mind that it is based on years of participation only, cannot use years of service.
When determining the lump sum for 74-307, 415 limit should be adhered to i.e. do not base the lump sum based on a very high mortality table that exceeds 415 lump sum at NRA. This lump sum is based on projected monthly benefit at NRA which can change from year to year.
If you continuously reduce the benefits and/or freeze the plan, you will need to recalculate the insurance coverage and make sure that incidental limits are not exceeded.
When applying RR 74-307, do not forget the 66.66% rule for whole life and 33.33% for universal/term life.
You can certainly flip flop using 100x or RR 74-307, no requirement that it has to be the same method to check.
However, as always, please check the plan document language and see what is allowed and what is not.
My 2 cents FWIW. FYI, I hate insurance in any pension plan, nothing but trouble and never explained by agents properly and what the consequences are, what do I know.