How do I determine which lens is best for my requirements?

• Use the formulas below either to find out how large your image will be when you know the distance from the projector to the screen, or to find out how far from the screen to place the projector if you want an image of a certain size. Since the size of the image can be changed by rotating the zoom ring, formulas are given for calculating both the minimum and maximum sizes. Note: The formulas given here are for when the projector is placed flat, without tilting it up at the screen. If you tilt it up by extending the feet or use electronic keystone correction, your image size may vary. Standard Lens (ELPLS01)
  To determine the diagonal size of an image when you know the projection distance:
  Inches:
  Maximum diagonal size = (0.676 x projection distance) + 2.293
  Minimum diagonal size = (0.483 x projection distance) + 1.639
  Centimeters:
  Maximum diagonal size = (0.676 x projection distance) + 0.903
  Minimum diagonal size = (0.483 x projection distance) + 0.645
  To determine the projection distance when you know the diagonal size of the screen image:
  Inches:
  Maximum projection distance = (2.070 x diagonal size) - 3.393
  Minimum projection distance = (1.480 x diagonal size) - 3.393
  Centimeters:
  Maximum projection distance = (2.070 x diagonal size) - 1.336
  Minimum projection distance = (1.480 x diagonal size) - 1.336
  Long Throw Zoom Lens (ELPLL02)
  To determine the diagonal size of an image when you know the projection distance:
  Inches:
  Maximum diagonal size = (0.487 x projection distance) + 2.3
  Minimum diagonal size = (0.286 x projection distance) + 1.5
  Centimeters:
  Maximum diagonal size = (0.487 x projection distance) + 5.9
  Minimum diagonal size = (0.286 x projection distance) + 3.7
  To determine the projection distance when you know the diagonal size of the screen image:
  Inches:
  Maximum projection distance = (3.492 x diagonal size) - 5.1
  Minimum projection distance = (2.053 x dsta