The levels are 3" apart. I copied Everard Junction's method, see
If you get your levels right then it's down to how good your Loco traction is. I've pulled a rake of 10 coaches but the Loco was protesting at that. 6 or 7 coaches should be no problem if you are pulling up the Helix rather than pushing. I would not recommend a Helix for Steam Locos as a rule but I did manage a rake of 6 coaches with my UTA Jeep.