There’s nothing to calculate.
These are the Unreal Units a.k.a. centimetres. This translates the (lets say) 200 pixels of widget screen-space into 12 meters of world space.
It’s up to you what values to punch in. If your world is 4x4km, you’d want -200k <-> +200k since 1km = 100.000cm - uus.
Clicking on the very left hand side of the widget would place the actor at -2km (Y) from the centre of the world origin.