mapping geometry question

Lets say I wanna make a 64x64 alcove centered perfectly in these weird angled walls. Is there a smart way to do this in GZDoombuilder? So far I've just disabled the grid and tried to stretch all the lines to 63-65 length and I am eyeing out the angles hoping their all parallel and perpindicular to each other.

Is there some kinda tool I could use that could possible assist me with this?

Split the wall in three so there's a 64 unit segment in the middle, then use the stair builder to make a sector 64 units deep into the void.

If you're having trouble getting it centred, you could first split the line in even halves, draw a 32 unit line out from the central vertex in either direction, then merge them together.

Mithran Denizen said:

Split the wall in three so there's a 64 unit segment in the middle,

I start with that method, then draw a 64x64 square on the grid in the void, then rotate it until it aligns with the wall.

1. Split the linedef in half.
2. Create your 64x64 alcove in the void, rotated as needed.
3. Split the north-western linedef of the alcove in half.
4. Select all 5 vertices the alcove has now.
5. Catch the selection by the 5th vertice.
6. Move the selection onto the linedef's middle vertice.
7. Once the linedefs and vertices snap together, you can delete the middle vertice.