It could also be worth exploring pixels sizes more widely either side. With my setup (an ATK428, smaller but with same size pixels) I get a good lock and calibration (RMS 0.13A) for 2x binned pixel sizes between 8.90 and 9.06 for a nominal 4.54×2 = 9.08. Using the 3123/(difference in pixels between H alpha and H beta) formula, I get 9.02 which is towards the top end of the lock range
(I was going to attach an example lamp image for comparison but max allowed size is 2Mb and zipped files are not allowed. Can you send me your image via email to robin_astro (at) hotmail.com please)
Robin