A custom scale is not designed necessarily to be a number, thought it will be stored within the database as a number for the purpose of ranking.
So to take one of the examples we use in BTEC grading the scale is
Refer, Pass, Merit, Distinction
The "maximum grade" will come from the total number of entries in this scale i.e. 4 because internally Moodle will store these values as
Refer=1,Pass=2,Merit=3,Distinction=4
For example you cannot get a Merit unless you have all the Passes, so simply adding up the Merit grades does not help calculate anything meaningful. So in this context each scale level, although stored as a number cannot be meaningfully used as a number.
It would help if you gave more context to your use of scales as I suspect this discussion thread is at cross purposes.
http://docs.moodle.org/24/en/Scales