FPGA development environments have power estimators that tell you for your particular design how much power it will consume. Be warned, this is a GIGO situation. The analysis tool will make some default assumptions about how often each pin and flip-flop will change state, if that is wrong the results may not be sensible.
Yea, I didn't have a lot of luck using these tools, but it may very well be that I didn't provide good enough inputs, and as you say, the results are only as good as the data you provide.
So the way I typically approach this is as follows - I use XPE (Xilinx Power Estimator) spreadsheet to figure out worst-case current consumption by setting artificially high utilization and switching frequency, then I design my board to this spec, but include hardware for current measurement. Once design is completed and verified in hardware, I measure actual current consumption in the worst possible regime (high die temperature, maximum allowed Vccint voltage), add a safety factor of 1.2 to 1.5 (depending on expected operational conditions) and change PDS to this specs.
But more often then not, I just leave things as they are for the worst-case. Because FPGA typically cost so much more than everything else on a board combined, that it makes little sense to spend additional time resizing PDS, because the savings might not be worth the effort. Exception to this is the case when DC-DC converter I use is a part of pin-compatible family with different current capability, as downsizing PDS in this case involves just loading another member of that family with lower current capability.