With help of this RFA you could measure your end point elevation with next inputs:
Point (Start Point) — your first point with known value
Slope () — your desired slope to achieve
Point (Final Point) — calculated value based on what you have at A. Could be positive or negative answer (see Visibility options)
Calculation logic for this small tool is simple:
A + (A * %) = B — for positive values
A - (A * %) = B — for negative values
OR A + (A * -%) = B if % slope given with negative value)
Press Space to quickly change displayed values locations in active viewport.