Well , in another thread we ranted a lot about the pitfalls of macro's.. I don't like them, but in a GCC environment like this there is not better alternative.
Still points up for writing down formulas in code, rather than the calculated values. It's more flexible, and the formula itself is already documentation rather than some "magic" number.
You could write the formula in a function, but then you're not creating a const setup value. A compiler can optimize this away, though. But using a macro will probably yield the most deterministic code in terms of performance, as it needs to be fully evaluated irrelevant of optimization levels.
If you only use this value at 1 location in your program (e.g. at initialization of peripherals), you could also do the calculation inside that function. Arguably a bit nicer than littering the program with more global variables.