Here in Australia FX-Universal is now labelled with black and white dilutions as well as the colour dilutions (well, my last 5Litre bottle did, anyway)
For black and white, they say 1+6 for film and fibre and RC paper, and 1+5 for replenishment.
I agree with the above post which recommends 1+4 on the grounds of more rapid processing and less frequent disposal. I did read somewhere that FX-Universal's capacity is less than for standard acidic rapid fixers. I don't know if it's true. I tend to not aim to get the last bit of use out of fixer. The low price makes it less painful to tip it out.
It's a nice fixer to use, since it is about neutral pH and doesn't smell much.