Sorry, I was speaking rather loosely. The first point I even consider is exactly hex 20. There are other points which should apparently be avoided in Windows Symbols Fonts. Here’s a comment from my code (with decimal character numbers):
// NB Do not use positions 127 or 160 (see Fontographer 4.1 manual p173)
// Now also do not use positions 127,128,129,141,142,143,144,157,158,160
// (see Macromedia knowledge base).
I have to confess that I started the Windows Symbol Font around 1991 when TrueType came out in Windows 3.1. It’s probably sacrilege around here but I used a program called Fontographer which I really liked. But it seems it was given up by its authors to a company called Macromedia. All development appeared to stop, they regarded it as a ‘cash cow’ I think, and it never AFAI-Knew-Until-A-Moment-Ago-When-I-Checked made it out of the world of 16 bit computing. Anyway the above information came from the manual and support forum.
[I see the Wikipedia article on it says ‘development was halted from 1991-2006’ (15 years!!!) so maybe it has been resuscitated, but in the mean time I discovered Font Creator, with which I am sufficiently impressed that I feel no desire to go back! ]
Anyway, the upshot is that I avoided (decimal) code points 0-31, 127,128,129,141,142,143,144,157,158,160 (and am nervous of 255!) leaving 213 of the possible 256 slots. And I filled most of them, so it’s definitely time to go to Unicode!
Your project is very interesting.
If you find time to post items to the Gallery section of this forum that would be of great interest.
I cannot justify this, but when I use the Private Use Area I tend to avoid U+F000..U+F0F just in case some software packages might make some assumption about the application being something about a Symbol font. Just me maybe, but I thought that I would mention it.
Well my font is a Symbol font at the moment, but the replacement Unicode font will use a selection from U+E000 to about U+E400 initially. My thoughts were vague but not entirely different from yours 
I’m new around here (though not to Font Creator) and I didn’t know about the gallery. I’ll take a look and consider what I might add.
David Webber