1/ when fitting the pivot post is it a 50/50 split i.e. middle of the vane or more like a 25/75 split for a longer trailing edge?
Generally a third in from the forward tip of the leading edge for full flying control surfaces.
2/ what will I need to set up the X tail radio wise? my transmitter has a V tail mixer program. shopping list required
If you're going to use a leveller (advised unless the boat is very slow) then mixing in the TX is non-starter- you will need to mix onboard. You can use an aeroplane v-tail mixer for this, but you can experience problems with these and levellers sometimes. Fortunately both Norbert Bruggen and Engel offer levellers which are designed exclusively for x-tail boats.
3/ how difficult are they to set up ?
If you mix electronically, then in mechanical terms, an x-tail is the same as a cruciform tail spun around 45 degrees. Space can be tight on modern submarine designs.
4/ can you use a sub leveler on them?
Yes, but see the answer I gave to question 2.
