[¤[4thPHiL]¤ Machalnik]

The equation is:
(#of players) X 8 X (maxrate) = (total Upstream in bits)
-or- in your case
(total upstream in bits/(# of players X 8) = (maxrate)

so convert your up stream first 256kbps=256,000bps. Then:
256,000/(8players X 8) = 4000
That's too low of a maxrate you need to be between 5000 and 8000 (yes too high can cause lag too!), so:
256,000/(6players X 8) = 5333

Not the best... but 6 players will get you into the 5000 range. Now another thing to consider is any other programs that maybe running on the server that might be grabbing a piece of your bandwidth (rogerwilco base, Teamspeak server etc). If you're running anything else you need to subtract a little from your total upstream.

My server is 1 MB (both ways) and I host a Teamspeak server too, so here's what I did. Instead of using the 1MB (100,000b) I used the values 786k (786,000b) for my calculation (just to be safe).

786,000/(12players X 8) = 8187.5

so I just set sv_maxRate to 8000 (it's not enough to squeeze in another 2 players so hell with it!)

...also it's a good idea to do a speed test (at someplace like dslreports.com) for a more accurate picture of you bandwidth because "residential" networks can loose a bit of bandwidth to overhead