Nice chair, the adjustable height is good but unless it is set up on a flat surface it will be unstable. Observing chairs really need to have three point contact with the ground, that enables them to be stable on a range of imperfect surfaces.
Also the width of the footprint relative to the maximum height of the chair will probable be an issue for stability.
There are several designs out there for observing seats that are very simple to make, probably the best is "The Denver Observers Seat". If you do a Google search I am sure you will find it.
http://www.schlatter.org/Dad/Astronomy/chair.htm
It is great to see people be creative and make their own stuff. I am sure that, after you use your seat you will be able to overcome any problems.
Well done
Cheers
Trevor