Doesn't look like much but that little bit of wall layout work is a full evening.
Esp, for an out of practice amateur with angles to cut and a 50 yr old room that's not quite square.
Compromises and choices must be made. i.e. "Which corners will be square and which will take up the slack?"
Actually was able to make it all up pretty well and keep the square corners genuinely square.. Just had to shave a wee bit of the layout down one side wall. One corner thrust by the side stairs is an inch or so smaller in one dimension.
Next up add ridiculous amount of blocking to the old part of the stage to stiffen things up. It's already pretty stiff but at times we get some serious loads up on it. Add the piano and some kids dancing around and... Well, you get the idea.
Then the wall plates (which are all marked off) will come up, cripples will get measured and cut and "Voila" it'll start looking like something.