Hi Thumpie,
This is my approach, copied from a thread by MaxJacob titled Standinding vs Sitting, 8 - 31 - 2008. There was also a similar thread started by bassistpatrick in May, 2009.
Standing or sitting I try to keep an arrow from my chest pointing towards the bridge or close to it. I also position the bass upright enough to have the strings forward about level with my face. The combined effect is to have my hands and arms doing the work in front of me and weighing back towards my body. My left elbow is forward of and lower than my shoulder to avoid neck and shoulder problems and give enough room between hand and shoulder for relaxed ease of shifting and vibrato. Also, standing or sitting, I set height of bass so that the nut is about 1 inch higher than the top of my left ear when using lower positions, which a compromise of posture between ease of use of left hand and ease of maintaining good bow contact points with the bow arm.
When standing, the bass leans towards my body so that the corner between back and upper rib is resting in my left groin and the bass balances around to the right so the left thumb carries little weight and shifting and vibrato are as free and relaxed as possible. If you check this stance out you will see that the arrow from your chest is pointing at the bridge and the French bow will just miss your trouser leg when playing on the E string.
When sitting I have the back corner (between rib and back) further across my body to the right and tucked in under my right ribs where the bass ribs are lower. The bass is angled in towards my right leg and body and leans back across my body towards the support of my left knee. Both my feet are on the floor and, at 5 feet 7 inches tall, I am half sitting half leaning on the stool (a taller person can sit right down on top of the stool) The lower right rib is against my right calf, the arrow still points towards the bridge and the bow on the E string still misses my trouser leg. The height of the nut is again 1 inch above the top of my left ear, the bow contact point reachable and the left arm free to shift and vibrate. Both arms still weigh in towards the body.
This seating position allows pretty good posture and works for me when practicing or doing orchestral playing but I would choose to sit differently to play high solo material. I would also choose a smaller bass than mine, with more dropped shoulders too!!!!!
Much depends on your height and shape as well as the size and shape of your bass, and whether your are a French or German user.
DP
Ps Using a block under your left foot is fine. I believe lifting your left knee too high risks distorting posture and putting too much strain on your lower back. Someone once told me your thighs should be hanging down at about 45 degrees to the floor when sitting on a stool for best posture. Can an Alexander teacher confirm this?