Car: 98' C280 with single pin style brake caliper

Piston angle position:
Looking in FastLanes tools catalog, I see a ‘brake piston angle gauge‘, to get the piston face indentation positioned at 20 degrees. I take it that the angle of the indentation on the face of the piston is important to how the pad contacts the disc surface. I also take it that if the piston angle is incorrect, this could cause brake squeal, as the angle the pad touches the disc is off. When I replaced the brake pads a few months back (but not disc), I may have rotated the pistons as I pressed them in. And since my car's rear brakes are squealing so bad, I thought I'd check to make sure the piston angle is not off before going through the expense of buying new disc and new pads to rid the squealing once and for all. Looking at the brake piston angle gauge' tool on FastLane, I'm not sure how the tool is inserted (inserted from the rear part of the caliper maybe???), and which 20degrees to check? To be more precise with my question; If I'm looking at the caliper mounted on the car, with the pads removed, which direction should the indentation be facing (down, back etc.)? Or lets say the passenger side caliper, facing the caliper kneeling and looking from the passenger side, pads out and I’m looking at the inner piston face, what direction should that indentation be facing?

Caliper rebuild kit:
I take it that the metal part shown in the kit below is merely a tool to install the rubber parts?



TIA
MarkC