Those on-line "design" tools can be a little misleading as they are intended to sell a particular product. There are many different seating solutions on the market. There are at least seven things to consider with respect to seats and platforms:
1. How big would you like the audience to be? Keep in mind that its not always wise to put in as many chairs as the room can hold and dont forget about big over-stuffed pillows for the younger viewers.
2. What kind of chairs will you be using? Will they be reclining loungers, folding theater seats, non-motion loungers with ottomans, bar stools, or perhaps a sleeper sofa? You should make this decision early in the game and stick to it. You may combine different seating types. For example, the back row may consist of folding theater chairs and the front row may have reclining loungers.
3. How much space will the chairs take up and how much room will they require to recline (assuming you choose recliners)? On a side note, I strongly recommend that you consider having at least one love seat in your configuration. Loveseats provide intimacy if you want to snuggle and they give you space to curl up your feet for added comfort.
4. Will ceiling height affect the platform height? As a rule of thumb, you dont want to have less than 6-8 (80 inches) of headroom.
5. Will there be enough space around the chairs to accommodate traffic patterns? Will there be any sconces or door swings protruding into the room? Sometimes it may be enough to change the swing of a door to achieve proper traffic flow within the theater.
6. Will you need to provide electricity to the platform for motorized chairs or step lighting? What about cabling for transducers (also called bass shakers)? This issue is especially important with centrally located free-floating platforms.
7. Last but not least, will you be able to comfortably view the screen from every seat in the house? This issue brings up the idea of sightlines. A sightline (attached) is created from the center of the viewers eye to the corners of the movie screen.
Assuming that you have been successful in your search for the ideal theater seat and that you know how many seats you will have, the next step will be to determine the dimensions of your platform. Most chair manufacturers will provide you with minimum space requirements for their chairs but keep in mind that these dimensions may vary once you assemble the chairs at home. Always allow a couple of extra inches when figuring out the total chair width. The depth of your platform may vary from six to eight feet, depending on your chairs space requirements. Try a simple experiment with your recliners. Have somebody (preferably tall) site in the chair and measure it in an open position, taking into account that the persons feet will probably overhang on the foot rest. Take note of your dimensions and keep in mind that you may want to be able to pass in between somebodys feet and the chair in front.
Many reclining and rocking models require a minimum wall clearance (a tall platform may act the same way as a wall in restricting chair motion). For a better listening experience, its best not to place the back row all the way against the wall. Room dimensions permitting, a centrally located platform is an ideal solution. It deals well with room constraints (such as door location) and it places the chairs in a good listening position. Room constraints may dictate a wall-to-wall seating platform with the chairs against the back wall. We could, of course make the platform deeper and offset the chairs from the wall but that will push the front row too close to the screen. The question, What is too close? comes to mind. One rule of thumb is that the distance from the front row to the screen should be no less than one and a half times the screen width.
Sightlines and ceiling height play the biggest role in determining your platform height. Ceiling height is a relatively straightforward calculation. Subtract the ceiling height from your platform height and you should come up with a minimum of 6-8 (although 6-6 is acceptable). Sightlines are a little more complicated. They should be drawn in scale on graph paper or, better yet, in a CAD (computer aided design) computer program. I have attached a sightline study created in a CAD program. Typically, you should place the center of the screen as close to eye height as possible. That may be difficult to accomplish with over-sized screens and with the need to place the center channel below the screen. Having to look up at picture for two hours is guaranteed to give you a serious pain in the neck. At the very least, it is a good idea to keep the bottom of the screen below eye level.
Unfortunately, every design is a form of compromise between ideal, theoretical solutions and real-life conditions. The secret is not to allow your project to wander into either extreme and dont panic when things dont turn out exactly as you imagined they would. You will still end up with a great home theater!