From Xojo Documentation

Read-Only Property (As Double )
DoubleValue = aiOSFont.Descent

Supported on Mobile (iOS).

The descent of the font. The descent of a font is the distance from the baseline to the bottom of the lowest descenders on the glyphs.

Sample Code

Var f As iOSFont = iOSFont.SystemFont(20) // 20-point
Var fontDescent As Double = f.Descent