Drop/Recess is the distance from the top to central point of the fitter.

Normally measure a shade height along the slant, it’s called slant height.

Measurements of Round Shade: Top Diameter x Bottom Diameter x Slant Height

Measurements of Oval and Rectangle Shade: Top Width/Top Length x Bottom Width/Bottom Length x Slant Height

Hexagon & Octagons Lampshade is Measured by Point to Point: Top x Bottom x Slant Height