My aircraft is at a strip which is not on the charts but is near two which are marked
The definition of "in the vicinity" is 10nm last time I looked. So if you a within 10nm of the other strips, you would use 126.7 (or the CTAF freq if different)
What about if the area frequency boundary went close to your strip?
One of the reasons the the MULTICOM (NB: in upper case) has an advantage over area is that you may always be
in the vicinity of some strip. The safest option is to have a common frequency for low level ops that is simple and easy to remember. Determining a frequency by whether it is on a chart or not only adds a degree of difficulty that was not there before. It is a simple procedure, especially if you don't know where the strips are? Oh!! And what chart was that??