On both of my SCBA set-ups I have two more valves, a gauge in between these two valve and a inline bleeder down stream of these just before the hose. I normally connect the gun, open the SCBA valve, close the bleeder, open the furthest downstream valve all the way and use the first valve as the fill control valve and monitor the fill pressure with the gauge between the two valves. I trust the larger easier to see gauge more than the guns gauge and I can control the flow very easily with this flow control valve. You can, and I have used the SCBA valve but like you said it's pretty easy to crack it just a little too far.
Jimmy