DOS Compatibility Card: Maximum Amount of Shared RAM

I have a Power Macintosh DOS Compatibility Card, and I am sharing memory from the Macintosh logic board. How does the PC Setup control panel determine which memory choices to make available?
Memory allocated in the PC Setup Control Panel is shared in the following increments: 2, 4, 8, 16, 32, and 64 MB. The PC Setup control panel allows you to choose any one of these settings with the following constraints:
Examples

RAM Installed on                PC Setup control Panel
Macintosh Logic Board           Offers
---------------------           ----------------------
8 MB                           2 and 4 MB
16 MB                           2, 4, and 8 MB
24 MB                           2, 4, and 8 MB
40 MB                           2, 4, 8, and 16 MB*

Note: one half of 40 MB is 20 MB and the next lowest increment is 16 MB.
Published Date: Feb 19, 2012