Yes, as I suspected, you removed the brake caliper, then removed the pad from the caliper so the rears could be measured. If checking the fronts by this same method, the brake pads can be slipped out of the caliper after gaining access of removing the wheel and retaining pins.
Using the digital caliper or similar measuring device, is the correct method, unless the brake is equipped with visual wear indicators, or audible wear indicators, to very accurately do as you have done to verify actual wear.
Typically though, folks will order new brake parts and while accomplishing this measurement or visual check, “just go ahead” and install new parts since it is already apart. With that they do surrender full life of the brakes.
The credit card or thickness gage method does not require any disassembly in most cases. It does however require that when checking, the brakes are clamping the disc to get a reasonably accurate measurement.the thickness gage method can be very accurate when done correctly, the credit card method, might allow a person to wear the brakes slightly more than published specs.
Two entirely different methods, both can work. Add to this, often a simple visual inspection, even if using an inspection mirror and flashlight, with a trained eye to know just what thickness remaining is needed works well too for a quick check, but certainly not best to obtain 100% brake wear.
Rather than doing the math and subtraction method you stated, next time you might consider to measure the thickness of the clean brake pad backing plate, and while measuring hit the “zero” button. Next measure the entire brake pad thickness. This gives a direct measurement of actual pad friction material remaining.
...I was going to post up something about the CC.
