In my experience with Bayer Disease Control for Roses etc, you get good control by just covering the top surfaces. I recommend that you try that to reduce the amount of fungicide used.

When I used sulfur, I was careful to spray both surfaces, because sulfur washes off easily and isn't super effective.

The great majority of particular local blackspot infections occur on the upper leaf surface. How can you tell the difference? Such spots do not usually bleed through to the lower surface. But spots that start on the lower surface do bleed through to the upper a few days later.

I spray a quick blast under then a good coating on top. Works fine for me. If you're roses look good, don't mess with anything. "If it aint broke, don't try to fix it."