How can I add to a number in project properties through context?

If I have a number in Project Customer Properties

${#Project#fpl_us48_single}

Which will pull the value of 11770

I want to add to this federal baseline number to meet different scearios (thus if the Federal # changes I don't have to change anything but the one property value)

So how could I +1 to this context?

Thank you,

Rob