BB,
There's no need for any mathematics or conversion tables. It's a simple 2-step procedure:
1) If in a country where QFE is used at military airfields (e.g. UK), use QFE at military airfields.
2) If elsewhere, use QNH.
Personally, I would rather use QNH at all times in the UK as well. Having 2 ways of saying the same thing is great if you're writing a novel, but it goes against principles of standardisation and seamless interoperability that the world of aviation generally subscribes to. It's not difficult to switch from QFE to QNH when operating abroad, but then in my humble opinion it's not difficult to fly a circuit on QNH either. Certainly not difficult enough to bother maintaining a national difference in standards.