There is a lot of guesswork with latency.
There are latencies in your cards A/D and D/A converters which probably
aren't quoted anywhere. Any SoftSynths or effects you use will have
internal latencies, which (although probably quite short) will add into
the overall - as will the jack latency of connecting one to the other.
External hardware synths, mixers etc. will all have latencies of their
own, some tiny some significant, many variable!
Having said that I do my work with latency in jack set to either 5.8 or
11.7. Only occasionally do I have to re-align tracks to take this into
account (usually when taking a MIDI controlled hardware synth through
JAMin).
As a side issue, JAMin has an unusually high internal latency, which it
HAS to have in order to do some of its magic. I would guess that many
LADSPA effects would have the same issues.
Finally, bear in mind that a real instrument like a cathedral organ has
a mechanical latency of at least 50mS, and if it has a 64ft register
this goes up to around 200mS, yet people can still play them very
effectively!
--
Will J Godfrey
http://www.musically.me.uk