You're definitely onto it. Whenever using electronic test equipment, relying on calculations adds a hefty margin for error or confusion. When you just flat don't have the correct instrument that is when you fall back on doing the math.
For some things it's usually your only practical choice, such as measuring very high current using a hefty .1 Ohm resistor and then doing the math. You use the .1 so that the math is simply a movement of decimal places.
Have a great day,