Seeing how it's a thyristor model with a variable power output I assume you are taliking about on manual at full power. Doesn't it have a dial on it or something similar to tell you what f/stop to use? Setting the asa to 100 and multiplying the f/stop indicated by the max distance should give a close value. Of course testing is best and if you can get a good flashmeter to check it with even better.
I hope I got that right, it's been a while since I've ever worried about such details. I usually just grab the flash meter and check things out like that.