It seems that this feature of Build is little, or not known at all. So, here is how I proceed to assign a specific angle to a sector, or a group of sectors :
1 - Just select your target with [Alt Gr].

2 - Rotate the mouse wheel while holding down the following buttons : [Shift Left] + [Ctrl Left] + [Alt] + [Left mouse button].
You will then see a small textbox at the bottom of the screen. Enter the desired angle (can be made negative by pressing [-] after writing the number).
If sprites are inside the selection, their angle will be automatically adapted.

3 - It is done ! You just still have to validate by deselecting with [Alt Gr].
