How do you wish to use the meth?
Do you want to use to combat the knock or as an extra fuel source??
That will kind of determine how far you can take it. Say you were running 93 pump gas and a 3.2 pulley and you were running out of injector, you could use the meth to basically add fuel and cool the temps down. Then you might get away with another 1-2º of timing. Figure on any small pulley if you can get away with 14-16º of timing you are doing pretty good, some cars may only like 12-13º. This was a bad crude example but sort of how it works.
I don't see the need for meth injections unless you plan to run down around a 3.0-3.2 pulley range because that point the supercharger is creating a ton of heat compressing that air and regular pump gas burns pretty hot and could create alot of knock.
