Matt -- if you mean XF-85, I can vouch for it, I've used it several times and it's a very flat clear with definitely no chalkiness. It slightly lightens the underlying tone, but that's endemic to the fact it's flat. Highly recommended!
M/TB379
PS: RLM paints were not actually flat, they had a low sheen (at least when they were fresh). MM enamels reproduce this to perfection. A close lustre would be Microscale Flat, which is notorious for not actually being flat, but is easy to use and cleans up with water; once again it's maybe slightly more sheen than called for. Hard to say, but I've used it on armour to enliven the finish, and it looks acceptable.