The X8 servos have more throw than other normal servos so I have found I have had to go inwards on the arms to keep the resolution good with these servos. I would definitely try the 18mm first, when I ran them in my E5s I had to go in to there to get my Vbar up to reasonable numbers, on 20mm I was well below the green levels for cyclic and collective. On my Protos Max 800 that I've moved the X8 into I've gone in to 16mm to get good resolution where stock is also recommended as 20mm.
//Dennis.