testing afr with and without meth at idle

[brute4x4]

i haven't gotten a WBo2 yet so i cant really tell how much my 50/50 water/meth is richening up my afr under WOT (aquamist was protuned).

so i just had a thought and am curious if this will work or not or if anyone has tried this method yet.

i would like to let my car idle and then turn my aquamist on test mode where it sprays full power and monitor how much it enriches my afr before the computer takes over and compensates.

or is there a similar way i can do this with say my factory narrowband o2 that only reads down to 11.2?

thanks.