How to use flash on Nikon FM?

I have discovered the other day that my Nikon flash SB-600 (that I normally use on my D750) not only fits in the FM but also triggers correctly with the shutter!

I want to try and use it now with some low speed film like ektar 100, but I have no idea how I would go about measuring light correctly with the flash… I very rarely use the flash on the D750, so I just know basic functions and go on Program when using it, letting the camera taking care of itself.

But with the FM I would have to set everything manually. For normal photos it’s pretty simple following the light meter in the viewfinder, but that does not take into consideration the flash afaik. In terms of exposure, I am aware of the sync speed that in this case seems to be 1/125s (orange number on the dial). Can you give me any tips on how to expose film correctly with a flash?