As I don't yet make distributions from my U.K. Personal Pension and will not know the value of the fund as of December 31st, I check the $0 - $50,000 box. If I take into consideration the total of my 'Foreign Deposit and Custodial Accounts' and the physical value of 'Other Foreign Assets' (which is U.K. Personal Pension only) then i'd be over the threshold. However, if I disregard the value of the pension account, (as the instructions so state - not taking any distribution and not knowing the value) then I'd be under the threshold amount.
I'd be interested to learn what others may do. File anyway as combined; over physically the threshold, or not file as there is no distribution & December 31st value unknown