tuning bgwriter in 8.4.2

We recently upgraded to 8.4.2 and I'm trying to make sure our bgwriter is working as well as it can. Based on:

# select * from pg_stat_bgwriter ;
checkpoints_timed | checkpoints_req | buffers_checkpoint | buffers_clean | maxwritten_clean | buffers_backend | buffers_alloc
-------------------+-----------------+--------------------+---------------+------------------+-----------------+---------------
804 | 2 | 39171885 | 22562 | 211 | 24759656 | 4488627
(1 row)

...I'm not sure that it is, because as I understand things, assuming I don't keep updating the same pages then buffers_backend should be a small percentage of buffers_alloc, and buffers_clean should be larger than it is compared to buffers_checkpoint. Is my understanding correct?

My checkpoints are spread exactly 5 minutes apart, and a typical checkpoint log entry looks like:

checkpoint complete: wrote 48289 buffers (3.7%); 0 transaction log file(s) added, 0 removed, 14 recycled; write=149.872 s, sync=0.378 s, total=150.256 s

The only bgwriter tunable we've adjusted so far is bgwriter_lru_maxpages = 500, though we've also set checkpoint_segments = 768 (not that we need it that high, but we have the space on the wal volume.)