There are many online forums for home theater systems where projectors are discussed in abundance.
And while there can be a lot of good information on those forum, it is also often relates specifically to home theater applications and is not always applicable to many professional/commercial projection applications.
Contrast ratio is a good example as it is the
image contrast ratio, which is the ratio of the light
level off the
screen between full white projected to the 'black' with the existing ambient lighting, that really matters and the contrast ratio of just about any modern
projector is not usually much of a concern in that regard unless you have virtually absolute
blackout ambient light conditions. The Digital Cinema Initiative specifications for cinemas
call for a relatively low reflected white
level of 14ftL and a very low reflected ambient light
level of 0.01 ftL, thus an
image contrast ratio of 1,400:1, a figure met by many modern projectors (apparently including their existing
projector). However, while that low of an ambient light
level may be feasible in home theaters it is very difficult to achieve in other venues, including theatres, without pretty extreme light control. As an example of just how much that can be relevant, the InfoComm
image contrast ratio standard currently undergoing public review for other AV applications identifies
image contrast ratios between 15:1 for non-critical applications to a maximum of 80:1 for critical applications. Those contrast ratios are well below just about anything you'll find for a
projector.
I do agree that many of the numbers for
projector brightness and contrast ratio are often pretty idealistic and are routinely based on ideal conditions that may not be valid in most real world applications. Put simply , assume that the
lumen and contrast ration values for a
projector are maximum values and that the actual values in use may be lower, especially after some hours of use as lamp output usually decreases over time.
Typically the most critical factors that you did not list would indeed be the
image size and
throw distance, the distance from the
projector to the
screen (or
scrim in this case). Also potentially relevant would be the relationship of the
projector to the
screen vertically and horizontally.