Maybe an example would help.

(Just FYI I use strobes.)

First, I decide on the DOF I want and set the aperture that will get me what I want, say f4 a classic short DOF portrait. This does not change once it has been set.

Second, I measure ambient light and set the shutter speed to get exactly the amount of ambient light I want, say 1/60th to allow the back ground in or 1/400th to keep the background out. (With strobes, as long as we are in the sync range, shutter speed has no effect on the main subject's exposure.) This setting also does not change once set.

Third, I measure and adjust the main strobe to match the camera aperture setting (f4). This is then left alone.

Forth, I measure and adjust the secondary strobe to get me the ratio I want, maybe f4 for a 1:1 fashion shot, f2.8 for a gentle 2:1, or f2 for 4:1.