Sorty about that : ( I find I can't go to close to my hip on my bum or right up close to my spine. When I first started using my bum it took me a couple of weeks to hit the right spot. For me, that's a band above my knickers and only a few centimetres high.
When I was using Teflon cannulas, I found I needed a longer length in my bum than my thigh or tummy (17mm as opposed to 13mm) Now I use a steel cannula. I used to have to increase my basal ever so slightly too, but I haven't had to do that this time.
Don't know if that helps, but I thought I'd say as I had problems to start with, but have gradually perfected my technique with experience.