Help -how to use new value

[somnoos]

Hello,
Please can someone show me how to automatically map and use the new values on a cell?

E.g. below, I want B7 to automatically use the new value 2500 and deduct whatever number I use on C7.

Please see image

 

[Micron]

In formula bar when A2 is selected type =B7-C7?
Specifically, that will use whatever value is in B7, not just 2500. If you want to use 2500 regardless of what is in B7 then
=2500-C7