Target Lambda with Water Methanol Injection

So I typically tune my Subaru to a Lambda value of 0.78 on C16, however, due to efficiency issues with the TMIC, I decided to add a Water/Meth injection kit to help reduce IAT.

Is there a recommended Lambda value I should target or will 0.78 be sufficient although tunes on pure methanol are usually around 0.68 Lambda?

Also, I set injection to begin just below the low boost setting in case of system failure the ECU switches to the low boost map.

First up I'd say that 0.78 is pretty conservative on C16 since the octane of the fuel is so high that you are unlikely to have issues with knock unless your boost pressure is very high. I normally tune in the range of 0.82 on C16/Q16 which is still what I'd consider to be quite safe.

You're right that pure methanol does typically get tuned at a richer lambda target (the likes of pro mod engines are actually in the vicinity of 0.55), however what you do with the lambda target will depend on the volume of methanol relative to C16. In most instances the actual methanol volume is relatively small so you don't need to go crazy rich. Given this, and the fact I don't know the specifics of your methanol volume, I'd suggest that your 0.78 is probably a reasonable place to start.