DB Plan with Unit Credit Funding Method hit FFL last year so all past bases are fully amortized. This year when using the previous assumptions and plan terms there is still no unfunded accrued liability therefore I do not create a gain/loss base. However, there was a plan amendment effective this year that does create an unfunded liability.

Is the base I establish for my amendment equal to the change in liability due to the amendment, or do I limit it to the unfunded liability so my equation of balance holds?

Yes and no.
The base for the plan amendment is the change in the unfunded accrued liability. If that is zero, so be it.

But don't forget about the credit balance. If you have one, then you will have to establish a new base in this amount, which the IRS reg. calls a "loss", to be amoritzed over 5 years (for 412 purposes). To be precise, this base is established before you do the plan amendment, so the equation of balance is maintained.

Base=UAL+CB+RA , assuming no OBRA FFC in prior year.

As a further clarification, the UAL should start at $0 when looking at the change. In other words, if the UAL prior to the amendment is ($100,000) and the amendment increases it to $25,000, the base that gets amortized is $25,000, not $125,000, (assuming no credit balance). If it's negative before and after the amendment, nothing is amortized.

Your balancing equation will not work until you have a positive UAL at which time you need to re-establish it as PAX & Libor said.

It can create some fairly "strange" results, but I believe it to be correct.

Yes! the UAL in my expression is the one Chamelnix refers to in his first paragragh - post amendment.

To be precise, the equation of balance mentioned here is really a subset of the correct equation:
PVB = PVFNC + sum(outstanding balance of amortization bases) + (Assets - Credit Balance)

See IRS Reg. 1.412©(3)-1(B)(1). http://www.access.gpo.gov/nara/cfr/cfrhtml...26cfrv5_00.html
(Of course, this reg. pre-dates the existence of the Reconciliation Account.)