Blender notes - resetting shape key value to basis

Use Blend From Shape:

1. Select all vertices whose positions you want to reset,
2. Execute Blend From Shape, accessible from W, Ctrl-V or Mesh > Vertices menu,
3. Set "Basis" as shape key to blend from (by default),
4. Set blend factor to the maximum value 1.0 (by default),
5. Uncheck Add, so only values from "Basis" will be blended in, effectively reverting all selected vertices' position.

Kind of wish there was a slightly less arseabout way to do it but a way is better than no way.