Without doing the math, how do you know your true altitude AGL when your altimeter is set to MSL
I usually don't care a lot about my AGL altitude.
When I'm doing my flight planning on the ground, I do take into account terrain and obstacles, and pick appropriate MSL altitudes to be well clear of anything on the ground.
But by the time I get in my plane, I'm thinking in terms of MSL (altimeter) and no longer about AGL.
That even applies to approaching my destination field. If I know the field is at 1,500 ft. and I want to do a traffic pattern at 1,000ft. AGL, then I plan for my altimeter to read 2,500 (MSL). And I think about if I'm at the proper MSL altitude (2,500), not what my AGL altitude is.