VBA Addition to existing cell

Hi All,

I'm trying to wirte some code that take the user input and then adds an element of code to the end of it so that it creates a foruma (example below);

Room Width = 10
Formula ideally would be = "Room_Width * UOM_Multiplier"

Essentially the cell value to become 25.4 but the formula within it to be "Room_Width * UOM_Multiplier" so that if I change the multiplier the value changes instantly.

I know I could simply make a formula to multiple by 25.4 or divide by 1 based on the UOM but when you have a very large number of calculations this is really slow.