I've read your original post and the second one you submitted, and it all sounds well. Some points to make:
1) When you installed the thermostat three months ago, did you rotate it to make sure that the bleeder (a hole with a jiggle piece, or sometimes an actual ball-valve) in the rim is positioned at the high point? If not, it's probably why the system "burped" (coolant dropped) the first time when the thermostat finally opened -- it released an air pocket that was initially trapped when you first filled the system, something the bleeder would have gradually released even before the engine warmed up. In any case, once the system has burped, the position of the bleeder is moot (unless you develop a leak), so don't worry about rotating the thermostat now -- just remember for next time.
2) You should (or at least could) have first let the engine warm up with the overflow tank's cap off (it won't explode water all over you, honest, because the coolant won't boil), and then added mixture to the max level before putting on the cap -- that's what I do; and I would suggest doing this now (including putting the heat temperature at "Max" (but the blower fan is unnecessary), just to make sure that the level is proper.
3) In any case, it doesn't seem like you have any leak, headgasket or otherwise, because the system seems to hold pressure for a reasonable amount of time after shutdown.

So just fill (as in #2), and relax.
Regards,