Making a Crate Sofa?

I'm in the process of building a crate style sofa. I have a major back and leg problem and this is the only style I can sit on and be comfortable. I have all the measurements for the outside frame, 76"L x 32"W x 25"H. However I don't have the seat height.

I built the whole living room set back in the 80's following a pattern from "This End Up". The seat cushions measure, 24"L x 29"D x 6"H, with the back cushions being 24"L x 15"D x 5 1/2"H. I was thinking 7 1/2 " from the bottom of the frame plus the 6" seat cushion would make it 13 1/2" high making the back cushion extend over and above the 25" frame by 3 1/2". Does that sound right?

If anyone in your group has a "This Ends Up" sofa and knows the exact seat height, I would appreciate them letting me know.

Sincerely,
Adriana from N.H.