The easiest way to accomplish this is to just repeat the original expression in the new location. QlikVies doesn't let you refer to the results of calculations in another object.
You may think that this is a bit too much to guarantee consistency in your document, if you have to keep repeating expressions in different locations and you're right. The best way forward is to store expressions that are used multiple times in your document in variables. You can do this in the variable overview or - better- in your Load script. That way you create a SPOF (single-point-of-failure) and not multiple ones. You can now edit your expression in a single location, and all instances will change at the same time and in the same way.