OK this is how I adjusted my idle... I am not saying it is an official way.
I too had problem of idling going down under 500rpm when warm, I also have a manual.

I bent the tab....see when the throttle cable attach on the TB, it is a tab that rest on some kind of pin. well I took a pair of pliers when engine was warm, and bent it in a little and after idle settled, kept adjusting it until I was happy.
Now it is idling fine at 750rpm....

I did that after I was sure that there was no exhaust leak (manifold bolt was not tight on the rear bolt) that there was no vacuum leak (usually high idle is the symptom for that) and after cleaning intake and adjusting TPS.

I know my IAC is acting up and some times it idles at 2 or 3000 rpm.... need to be changed but it is working for now...


So to sumarize... look into that little tab bending.....
(I can probably post pictures if my description is too vague )