Move to sd doesn't use your ext partition. That's just the same as you get with a stock ROM. If you've been using that to move over apps, you have actually been moving them off your ext partition and onto the fat32 partition (inside .android_secure).
If the a2sd script is working, "move to phone" will move an app to ext. How much of your ext partition is used? You have 450MB free, so if it's a 512MB partition you've used a little but not much - possibly because you've been moving apps off it! Try moving them back to phone and see how it changes. Also check you don't have some setting to install apps on sd by default - you want it to be automatic or install to phone (should be OK by default, but if you've used something to override this you need to undo that).
Other things you can do: MIUI uses DarkTremor a2sd, so you can control that via a terminal emulator. Start a terminal, and type "su" to get superuser access. There are then a number of commands you can type, e.g.
a2sd reinstall - will re-initialise DarkTremor a2sd. Use this if it looks like it really isn't working (e.g. free space on the ext partition does not decrease when apps are moved to phone)
a2sd cachesd - will move the dalvik cache to sd-ext. If you are short of space, this might make a big difference (several 10s of MB).
In both of these cases the phone will reboot after you type the command - this is normal.
I expect that if you type "a2sd help" or "a2sd ?" it will list the available commands (or googling for darktremor commands will find them quickly enough). I used to know them, but haven't used an ext partition for a few months so don't have a copy of this script on my phone any more to check.