Yep, no speed backing here! Take your time, it's not like being at the landing where folks get ticked off if you take 5 seconds too long.
Know how much space you have between your trailer fenders and the door frame. As previously stated, back to your left so you can see the relation of your trailer fender to the door frame.
Let's say you have 4 inches clearance on each side of your trailer fender to the door frame. If you can see your left fender, favor the left door frame that way it's a given that you will have plenty of room between your right fender and door frame.
The other thing I do is I have a yellow wheel chock that I back the rear (2 axles) trailer tire up against as my boat fits in my garage literally by a few inches.
If I can see that the yellow wheel chock is in line with my trailer tire, I'm good to go.
Just thought of this. Go to Home Depot or wherever and by yellow some caution tape. Stick that to the floor on the left side of the boat and use that as your guide. If you don't deviate from the caution tape you'r home free.
For fact, I'm gonna get some of that today for my garage!