This is just a thought. Get your rifle with scope and front rest or bipod set up and then get into your normal comfortable shooting position and looking at the target. Make sure your buttstock is adjusted to your comfortable position. Once you are in place with the stock in shoulder, have someone measure from your stock where you would have a bag in place. That would be how I would start.

Another direction to take would be to call a bag manufacturer and see what words of wisdom they might have.