When a line item with a formula is mapped to a takeoff, the mapping supersedes the formula. To get around this, you can use the multiplier, which will adjust the takeoff quantity, or you can use a formula that is tied to a variable from the Project Tab and map one of the QTO Properties to the variable rather than directly to the quantity.
Using the Multiplier: You can adjust the multiplier and that will update the count.
Using a Variable tied to a formula:
Example:
Variable: Room1 = 25
QTO Property Count = 10
Formula: Room1*Count
Changes made to the variable update the Quantity.