hatch spar location

I'm building my first teardrop, just starting actually. I have the template done edges sanded smooth, I was laying out the cabinets and things when I began thinking about where should the hatch start. How do I determine where my hatch needs to start? I'm using a 48" radius on the back, I'm building a 5x10. I would greatly appreciate any help.
Thank you, Jeff
Thank you, Jeff