Net values for objects can be manipulated manually by clicking them and entering a mathematical correct formula. This can be a quick way to remove for example door openings or windows from the net surface or more advanced like adding angles to get the correct amount from a 2D measurement for roofs and slopes.
Clicking the button Angles will bring up a dialogue in which you enter how many degrees the surface is leaning and which value you wish to apply the angle to. Geometra will add the appropriate formula to the net value.
When changes are made to the net value they are displayed as green to make you aware that the value has been manipulated. To reset it, click the value and erase the contents.