Your method seems right, I didn't check the math. Sounds like the tank you have is sufficient.
Unless you've got some trapped air in the lines (air will expand MUCH more than the water with temp changes), and you've ruled out the autofill valve and heat exchanger in the indirect, I can't think of anything else. Try to purge the lines and double-check your air extraction device for proper operation.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013
Turn off the autofill for a week and see if the pressure stabilizes. If it does, you've found the problem.