We need the ICAO semicircular cruising level diagram which is on all the HI level charts – it’s on the front of the E(HI)4/5, or in the middle of the E(HI)2. (It’s the little diagram, not the larger 'metric' one.

We need an EVEN level above FL400, excluding RVSM .... look at the diagram, we need the right hand circle (at or above FL290) .... look at the EVEN side .... levels available ... 310, 350, 390 etc ... next in sequence FL430.

Don't worry about the 'maths' - 430 doesn't seem EVEN - but it is simply convention that refers to them as EVEN - ICAO, above FL290, there are no 'mathematically' even levels - that is because above FL290, the minimum separation criteria between opposite traffic is 2000ft, hence the minimum separation between traffic in the same direction is 4000ft.