I have created a "mileage expense" formula field which calculated the "miles" field by the mileage reimbursement rate - $.56 - hard coded into the formula.
Mileage reimbursement rates change at least annually based on the federal allowance for business travel.
My problem is two-fold:
Can I create a "constant" field to which I can refer in the formula for the "mileage expense" field? (instead of hard coding the value into the formula)
Can the formula be structured to "look up" the mileage rate "constant" and then leave the field's calculated value unchanged when the "constant" is changed? In other words, I do not want the calculated field value for 2014 mileage expense to be recalculated when the "constant" is changed for the 2015 tax year?
If that is not possible, I guess I would have to create a lookup matrix with the mileage rates based on date range, then calculate the field based on the rate in effect at the time of the travel. (And how would I do that?)