I have used an IMAX 2000 at various heights, 16', 20', 25', 32' at the base of the antenna. It is currently at 38' AGL at the base of the antenna. It is currently strapped to a 70' Redwood Tree up in between the branches next to the trunk and it works best for me on the 10 and 15 meter bands. (The mast goes from the ground up along the trunk) It will do some other bands, but it is not really resonant and a tuner would be shenanigans to make it work out of band. I use other antennas resonant for other bands.
I have talked around the world with that antenna at various heights using no more than 100 watts. I do not use the ground plane kit nor an RF choke. There is an inline lightning arrester on the coax at the base connected up to a copper grounding rod. There's another 50' run of coax into the shack.
Everyone's geographical location is different. If you can get it up above nearby buildings you're on your way. The sun is not cooperating right now and it will be a few years before we begin to climb out of this low, so do not be discouraged by poor reception. It is poor reception and it is affecting all of us.
If you want a height to shoot for, go for around 16' or 32' AGL at the base of the antenna. That antenna definitely moves in the wind, so ensure it is quite secure at the base and whatever it is mounted to is secure. Oh yeah, I used coax tape where the parts join and sprayed the antenna with a UV inhibitor paint/coating (no lead). The sun it will eat it up the antenna eventually.
* I have never heard of that multiples of 9 thing for this antenna.
