Finding the proper backfocus with the parts you already have and the accessories you want to use isn't hard, but with so many combinations, it can start to make your head spin. Use this free google spreadsheet to quickly try out different combinations and find the right solution for your next astrophotography shot:
https://docs.google.com/spreadsheets/d/1XTfgQZ5VXrAp3snv375iIY5AGC8pp8ARWHSln2rmAJg/copy