In high school, I was taught I had to know about 40 rules about the relationships between sides and angles in right triangles. Then I learned there’s really just one rule - SOHCAHTOA. From that, derive whatever you need on the fly.
Edited to add:
I should have said that you can ignore spherical geometry (or the more complex geoid) over short distances. Unlike the idiotic flat earth arguments seen on Youtube, the Earth is not so huge that the curvature isn't readily visible. For instance, Redfish Island in Galveston Bay has a peak elevation of about 40 feet. Before you sail from Redfish to Kemah, about 10 miles away, Redfish has sunk below the curve of the Earth. The Earth isn't just a little curved, it's nowhere near flat.
But, if your distances are, say, a half mile or less, you'll get acceptable accuracy by pretending, just for a moment, that the Earth really is a big dinner plate. Just don't admit that to your kids.
