There's a bit of detail missing from books about how to physically build the door, so I figured I would give some insight as to my process.
It started with the door. It's a steel door with fixed dimensions. Since I was framing out the space in which it would live, I could work from the door out. Verses trying to squeeze a door into a pre-existing space.
So, lets work with a hypothetical door and walk through the dimensions I chose. The door is 36" wide, I want a gap on both sides of 3/16", then a heavy duty frame of dimensional lumber which is 1.5" thick then a 1/4" gap on both sides.
so from the door, we have 36+(3/16+3/16)+(1.5+1.5)+(1/4+1/4) and doing some addition we come up with 36+3/4+3+1/2= 39 1/4 for a rough opening. so as long as the world is perfectly plumb and wonderful, that would work. I gave myself an additional 1/4 inch for breathing room So I framed out my rough opening to be 39 1/2" wide.
Now for the height there were a few things to figure. The room just had a raw plywood subfloor. I have some leftover floating floor from the living room project. So I needed to calculate how high off the floor I wanted things and still end up with a tight seal on the threshold.
Here's what I drew up.
So from the subfloor working up. We have underlayment felt that is 1/8" thick, flooring that is 1/4" thick, a 1/2" threshold and a door gap of 3/16". Then the door, another 3/16" gap, a 1.5" top door frame then a 1/4" gap between the door frame and the roughed out wall framing.
The floating floor will not be physically joined to the spacer under the threshold, a small bead of silent seal will act as a cushion. It will have some movement as people walk on the floor but for me the main priority was that it wasn't going to transmit vibrations from the other side of the door. The caulk will be hidden from view as the flooring will tuck under the threshold.
The door will also have a drop seal installed, so once it closes, there will be two seals on the bottom of the door and then the seals around the edges.