Simple Conditional Request Please?

[Okiewan]

Okay...

This conditional inserts code after each first post in a thread.
What I'd like to do is modify it, to add another piece of code a few posts down in the same thread. Ie; "insert after first post, then insert after 6th post"

Or... after EVERY "x"th reply in a thread.

The current conditional:

PHP Code:

<if condition="$post[postid] == $GLOBALS['FIRSTPOSTID']"> 


Any help? Thanks in advance!