You will see many artists holding bass guitars in completely different ways, particularly while standing. Sometimes the height of the bass or the length of the strap appears to be dictated by the style of music.
The important thing is to minimise strain and avoid any sharp angles on any of your joints which will lead to medical problems over time. In all positions your limbs should be relaxed with joints not clamped together or forced straight but at a relaxed angle. Particular regard should be paid to both wrists as hand problems can be created over time if they are under strain.
Sitting When sitting you need to have your legs at a right angle to your body. This can be achieved either by having a chair at a height that enables you to get to the correct angle with your feete on the floor or a higher chair with a suitable foot rest. Your bass should not be flat against your body, the neck should protrude at a shallow angle so that your arm from the shoulder down is not hard against your body.
Standing and Strap length Once you have a comfortable sitting position you should put the strap over your shoulder and adjust is so that the bass is still resting on your leg but there is no slack in the strap. Then when you stand the bass should stay in almost exactly the same place as when you were sitting.