This website might help a little.
In a nutshell set your sag up for you and your gear sitting on the bike.
Get a buddy to measure. THen set up you compression and rebound to your
feel. THe results you get for the sag measurements will let
you know if you need new springs. Once you know that and have
your sag setting you
can tweak sag if you want for occasional loads or fine tune
your handling. On the compression and rebound clickers
remember to never crank them tight (turn gently) and always
count from full in. So a setting might be "4 clicks out" which is
to say 4 clicks out from all the way in. In is usually clockwise
by the way. SOme bikes are different like the PDS KTM rears
but I don't think the 640 is PDS. FOr setting compression and
rebound start with a reasonable setup like the factory recommended
or maybe 1/2 way out on compression and 1/3 of the way out in rebound. Then go to a short section of trail or whatever that is like what you want
to tune (optimize for). Ride over the section and try to notice
what you are feeling. Make an adjustment and ride the section again
and see what happens. If the section is short it's easy to compare settings.
Too long and you forget what the last setting felt like. If you get your
setting where you like them and sag is correct you can tweak your handling
by small rear sag (couple mm) changes and or small changes in the fork tube height. Raising the forks 5mm in the clamps up front makes a big change in how things turn. Remember to torque the clamps correctly and if you
want more that 7-10 mm correction up front look around because its getting a bit much. I move mine usually no more than 5mm. Tons of other info on the web as well.