Custom formatting of a field is not maintained when used by another field with an Arithmetic Source of Data. In order to accomplish this, create a phantom field (Enable Printing = NO) that copies the custom formatted field, and use this field in the Arithmetic field.

For example:
TEXT1 = Phantom field, Date with custom formatting of "d - Weekday" to assign a number to the day of the week
TEXT2 = Phantom field, Copy of TEXT1
TEXT3 = Arithmetic field with this expression: SS("abcdefg",TEXT2,1)

TEXT3 will display a for Sunday, b for Monday, etc....

SIDE NOTE: Copied fields with Enable Printing = NO will maintain the special formatting, while Enable Printing = YES will not.