Is it correct that you must use prior year's comp for boyr val and therefore if you have a p/s plan alongside the db plan;you must therefore use current yrs comp and cant use beg yr val?
ty

No.

At the end of each year, you look at the actual history for several items:
PS contribution limits - 6%, or 25%, or 415 limit.
401(a)(4) non-discrimination tests on actual benefits earned in both plans.

But: DB contribution is based on actuary's best estimate of future benefits. If the DB val is done at beginning of year,
this just gives you a budget sooner, so you can plan for your allowable PS cost.