Warren, I use an adjustable secretary chair that I keep at about 18 inches.
You want to sit so that both of your feet are comfortable and you're not having to put any weight on your feet and your thighs are comfortably straight. Then when you put your left foot on the foot rest, it should lift your knee high enough that your guitar won't slide off of your leg.
From what your are saying, you might be happier with one of the ergo type guitar lifts where you can keep both feet on the floor. You may need to just experiment and find what works best for you.
Bottom line: You need to be comfortable, because if you aren't it's going to be tough to sit for any time and practice.
Music touches the heart, but playing classical guitar can lift your heart and enrich your life.