How to set Constrained Angles while drawing and holding shift?

Rotate the canvas using the rotate view tool (R) by whatever angle you need, then when you draw straight lines using the shift key it will be at the angle you have rotated to, then rotate the view back to 0 again.

Are yopu doing this with the brush tool or line tool?

Are you using a Windows or Mac system?

If you are using the Line tool and hold down Shift while dragging out the line, it will snap to 90° or 45° depending on which is nearest.

Another option is to create your lines perpendicular to the document borders, and use Free Transform to rotate to a precise angle. Note that I have used the grid on the left end of the Options bar to designate the point the line should rotate around.  I find this a bit small and fiddly nowadays.

If you are using the brush tool, then you have the option of using the grid.

Click at the start of the line on a grid intersection, and hold shift as you click on the second grid intersection. 

You can continuously relocate the grid origin by dragging from the corner of the rulers.

Tip: my preferred method of changing the grid spacing is to us Image Size with Resample turned off. i.e. doubling the Resolution in the Image Size dialog will halve the grid density.

I asked what platform you are using. If it is Windows you have the option of using the Lazy Nezumi Pro plugin.  LNP has dozens of features, one of which is to constrain the angle.

Question:

Is there a reason you are using Photoshop and not Illustrator?

Unless I'm misunderstanding what you are doing, holding the shift key for straight lines is not new. However, to draw continuous lines at angles, I would use the Pen tool, create a path, and add a stroke to the path (an option on the Paths panel). The shift key constrains the path to 45 degrees. You can keep the path to change the stroke settings later on if necessary.

This is the same general technique I would use in Illustrator, except the path stroke is handled differently.

Here is a link to an Illustrator symbol library:

https://www.codezebra.com/ElectricalSymbols.php

I don't know if this would help your needs, but here is a link to a CAD plugin:

https://www.hotdoor.com/

Here is the Illustrator setting for setting smart guides at various angles: