http://mewiki.project357.com/wiki/X264_Settings
insert into advanced settings.
CPU: i5 3570k stock, 852x480, 700kbps (fuck w/e going on @ encoding's "quality balance")
crf=5 me=umh deblock=2:-2 trellis=1 merange=24 bframes=16 b-adapt=1 psy-rd=0.1:0.4 aq-strength=0.5 qcomp=0.2 cplxblur=0 qblur=0 subme=9 nr=1000
imo, this is close to using the "slow" preset.
http://forum.doom9.org/showthread.php?t=148149
You'll have to mess around with the values for your resolution/crf value/max bitrate/cpu constraints. The example ^ is flaky. I used local recordings to get to my result, results might be different with the flash decoder though.