I am sorry that you have been waiting for a response. I recently just logged onto the site and noted that your question hasn't yet been answered. I am a licensed veterinarian with over 25 years’ experience and would be happy work with you if you are still needing assistance.
From you description, it sounds as if you are doing it exactly as I would. I can tell you that on some cats, you do have to exert a moderate amount of pressure in order to get things started, but if it seems like too much, I would give it a rest and try again after a short time. Expressing the bladder requires relaxation of the urethral sphincter and if she is nervous or tense, that may not happen readily. You can damage the bladder if you squeeze too hard and the sphincter is not relaxing. It is hard to describe what is too hard, but the best estimate is when Butter starts to object as if it is uncomfortable. If it seems she is and no urine is flowing yet. Relax your grip and message the abdomen a bit to help relax her and then try again.
Your positioning is as I would do, but I have come across alternative positions that I though I would pass along. It really depends on the cat itself and what you are comfortable with, but these may be things you could try.
This one is much like you do it: video 1
Here is an alternative position: video 2
And yet another position: video 3
Finally an odd one (that I don't think many cats would handle well): video 4
I hope you find this helpful. It sounds as if you already are doing a pretty good job.
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.