Backupserver requires ~1MB of shared memory for every active dump stripe soooo, you need to find out how many concurrent stripes (dumps and loads) could be in flight at any one time then multiply by 1MB and use this to set the backupserver's memory setting (-m <maxMB>). [I'd probably go ahead and add some extra space just to be sure, eg, for a max of 20 concurrent stripes I would probably set to 30MB or 40MB.]
I'd recommend the linux admin to set the system shared memory max at/near 20GB; this then leaves plenty of room for the dataserver (10GB) and backupserver (20MB-100MB ?), plus allows you to increase dataserver/backupserver memory without having to get the linux admin involved.