My projector is projecting an image that is getting darker and darker

Since you have a projection TV it has to be getting up there in age. Like a movie projector they use a halogen lamp to provide the image for your screen. With age they began to peter out and you picture becomes less focused and darker. Unless you have replaced the bulb in the last year or two its probably time to do it. Eventually there will be no picture at all.

Short Answer:
The distance calculation is the diagonal size of the screen times the throw ratio of the projector (x/12 for feet).
If the projector has a physical zoom this plays in as well, throw ratios are usually displayed at 1x or for the whole range (e.g. 82C; 1.2 zoom means 1.43-1.71 throw).

Details:
Light calculations are somewhat inside the scope of this question, but suffice it to say, they aren't fun to do IMHO which is why I like the online calcs for a quick reference. Rule of thumb, the brighter the room or bigger the picture, the brighter the need.

If you don't want to do the calculations yourself, there are hundreds of calculators out there. Projector Central has a good distance calculator that is preset for actual projector specs. I use it often so I don't have to mess with the calculations. Just go to their website and select "Calculator Pro" at the top. It even does estimated light readings (we aim for 80fL because of the bright room environment we have to deal with, but for controlled lighting 30-50 is a good range too.)
There are also apps available on Android and iOS like AV Tools or Alford Calc.

If you want to do precise calculations, you need to know the size of your screen, the throw ratio of the projector and the brightness of the projector (and the brightness of the room) and the reflectivity of the surface (gain).
The latter two of these deal with picture intensity, the fact as your image gets smaller, the image gets brighter; to a point that it can be uncomfortable to watch. The opposite is also true, that as the the image gets larger, it gets darker since the amount of light coming from the projector doesn't change.
And finally, the more ambient light in the room, the darker or more faded the image will appear as it competes with other light sources.

if it is a DLP projector, then it could be the color wheel.

No to your question. The lens is fixed it cannot shift horizontally only although there are projectors that can do this. I am sure you have the front foot retracted? I have had to do the same as you are doing or find a lower table.

if the cables are new, reset the settings to factory default to make sure the resolution matches. if there ia still an issue, replace the main board.

if the hazing is darker than the image, the light tunnel is probably collapsing and needs replacement.

The normal lifetime for almost all lamps for projectors are 2500-3000 hours in EKO-mode and about 1500-2000 hours in normal-mode

You will notice that the lamp is goint out of time by the light of it, the image will be darker and darker untill the lamp dies/explodes.

If your image isen clear you have to clean your projector, and by that i mean a realy good clean inside it. If you arent sure how to do it i sugest you leave it to a repairshop to do the work, its not expensive and they will do it properly.

Btw, if you ned to buy a new lamp, let me know, i know where you can order a new lamp to realy good price.

Good luck
Nic

The yellow you see is all dust on the mirrrors inside your projector.
If u are lucky you can get the mirrors cleaned by a pro, if you are unlycky as i was, the blue mirror has burn and u need to replace it.

Take your projector to someone who repaire tv:s ot other electronics, they will help you. If its only the cleaning its cheap, if you need to replace a mirror its expensive.

Good luck

What kind of computer are you using and what are the connections to the projector. 

find the menu for saving the settings, for sure there is a menu saving all your desired settings. good luck!