use a method called scribing when coming up against walls.

[MistressEll]

this can be accomplished using a scribing tool, or even a cheap item called a compass.

Sometimes one needs to use an “offset” in addition – a “given” straight edge as a reference point, to transfer the scribed cut-line to the length of the board.

You should also find a reference for square before you begin and snap some chalk lines on the floor, not work from an “unknown” like a wall edge.

Finally, you can always scribe a pattern, using for example some stiff paper or builder’s felt, then trim the edge towards your work using a scribe and straight edge to shorten your pattern to line up with your straight work already laid.

Then transfer the scribed edge to your plank using the scribe tool of choice.

For determining angles from a straight plane one can use a simple plastic protractor. Then you’ll have to bisect the angle to adjust for your miter saw.