You won't be able to add this to the automatic calculations. Those dump their result into the trkMax field. Are you doing this calculation to get a number of uses for a particular feature? That's what the trkMax field is for.
So instead of using the automatic calculations, do it with an eval script. I'd set the phase so time time Post-Attributes or later. Instead of dumping it into abValue (which is just a free-floating variable for use in abilities and their scripts), send it to that trkMax field.
Code:
field[trkMax].value = 1 + round(#attrmod[aCHA]/2,0,-1)