How can I determine the memory used by a forked process (Linux)?

Proportional set size is probably the best figure of memory use per-process you have available. It's the unique set size + (the shared pages / # of processes sharing).

So one process has used 44176 pages, of which 19388 are unique to that process. This means that 24788 are shared with others.

Looking at PSS as USS + (shared weight). The weight in this case is 8193, which is the proportion of shared memory pages (24788) attributable to this process. The more instances of an application you have sharing pages, the lower the weight for each process.

You can sum PSS across multiple processes and you won't count shared pages more than once. You can't, however, sum USS and PSS and expect RSS because PSS already includes USS.