The MTX memory map in "ROM mode" has room for 16 pages of 32k and then 16k fixed memory. The MTX memory test has a bug if all 16 pages are full, and reports no paged ram found ram as a result. So MFX maps in 15 full pages and one half page. Since there is already some memory in that area it actually means, either 32k or 64k of the expansion RAM is unused.
In "RAM mode" the pages are 48k in size, again with 16k fixed. the 480k of the 512k is used to create 10 full 48k pages in pages 1 to 10. In a MTX 500 the remaining 32k fills the gap in page 0 to give CPM it's full 64k compliment. On the MTX 512 that 32k is allocated to partially fill page 11 instead.
So either system has 512k in basic, in CPM the MTX 500 has 544k (64k base + 10 x 48k paged), the MTX 512 has 576k (64k base + 10 x 48k & 1x 32k paged). The CPM boot code inherited from ReMemotech/ReMemorizor allows for 256k or 320k Ram drive F: to be created in that paged memory in CPM.
The screen shot below is from the Excel spread sheet I was using to visualise the movement of RAM while working on the GAL code. The way the blocks move is to match the original Memotech scheme. That makes for some seriouly messy equations, that thankfully WinCUPL can optiminse, even then the chip select logic hits 16 terms.
16 is the maximum terms available on the largest cell in the 22V10. Which is why there's no jumper option for the MTX512S2 or RS128.