Dining are that turns into sofa bed question

Yup me again
Trying to do as much pre work before the trailer shows up late March. Today I'm working on a dining area that will pull out to a couch
http://vandogtraveller.com/expanding-bed-for-van/
I'm trying to calculate the depth, and height of a standard dinette frame. I'm thinking about 28" deep about 20" tall including the cushions that you sit on. And length will be no problem. Has anyone built their own dining seats and if so am I in the ballpark for measurements. The other consideration is how tall a cushion should be that you place your back against.
Thoughts?
Cheers
Al
Trying to do as much pre work before the trailer shows up late March. Today I'm working on a dining area that will pull out to a couch
http://vandogtraveller.com/expanding-bed-for-van/
I'm trying to calculate the depth, and height of a standard dinette frame. I'm thinking about 28" deep about 20" tall including the cushions that you sit on. And length will be no problem. Has anyone built their own dining seats and if so am I in the ballpark for measurements. The other consideration is how tall a cushion should be that you place your back against.
Thoughts?
Cheers
Al