BSG fairly well described a bit the way to find the combination. Will come back to that in a moment.
A common thing to do these days is to set the combination to 50. Turn the dial counter-clockwise four full turns, stopping at 50. Then turn clockwise until the lever nose drops into the drive cam. If that is the correct combination, the lever will go all the way down, with the fence (flat bar that crosses over all the wheels) dropping into the gates (cutouts in the wheels). The dial will come to a stop, but the bolt will not retract with the back cover off due to the internal relocker. If the dial stops turning, then you can press the back cover on then continue to turn the dial to retract the dial.
To find the combination if the fore mentioned test combinations, turn the dial clockwise at least four turns until you see all the wheels starting to turn. When the gate on the wheel closest to the front of the safe is lined up under the fence, look at the number on the dial and write it down. That is your first number of the combination.
Now turn the dial counter clockwise until the middle wheel's gate is under the fence. Look at your dial and write the number down. This is the second number of your combination.
Now turn the dial counter-clockwise until the gate of the wheel closest to you is under the fence. Look at the dial and write down the final number of the combination.
Finally, turn the dial clockwise until the nose of the lever drops into the cutout in the drive cam.
If the lever does drop into the drop zone (cutout in the drive cam), you have the correct combination. Put on the back cover and you are set. If not, the. Look carefully at the fence and wheels. You will likely see that the gate for one of the wheels is slightly out of alignment with the fence. The wheel closest to the front of the safe door is your first number, the one closest to the back cover is the last number. Find which gate is out of alignment with the fence, and which way it is turned.
Seen from the back of the lock, if the gate is too far to the left of the fence, then add one (or more) numbers to that combination number. For example, if your first combination was 20-95-17, and the gate of the middle wheel was slightly off to the left, then try dialing 20-96-17.
If the same gate is off to the right, then subtract a number. 20-95-17 becomes 20-94-17.
Of the gate is still off, then adjust the numbers accordingly.
Let us know how it goes for you! You don't need to tell us the combination, just that you got it.

Gordon
Just when you finally think you have learned it all, that is when you learn that you don't know anything yet.