Ah, The slant. Dats why it is hard to mark and return to suspended fish accurately. If the bait fish are close to surface, you gonna be close. If the are deep, not much so. 2D is you best view when returning to marked suspended fish or bait fish. SI is relative to the bottom. When marking, I mark at the base of the object (opposite the shadow). Inches count. Always remember that everything is relative to the transducer. If your transducer is off to one side, the center of the water column is relative to the transducer, not the center of the boat.

Just some stuff to think about while working your Pythagorean calculator.