Once mounted on an "I" adapter, the LNB should be a similar distance from the dish as if it's on a "Y" adapter.
Simple measurements of an "I" won't do much good, unless you document how far onto the arm it slides, and how deep into the other end the LNB sits.
A decent way to measure, would be to put a Twin LNB on a Y, and mount it to the Dish500.
Now, put a ruler across the face of the two LNBs (the white caps)
Measure with a string, from the center of the dish to the edge of the ruler on the side against the LNB faces.
Mount your single LNB at the same distance from the center of the dish.
...or better yet, just order one, and be done with it!
I don't like the outrageous price (plus shipping) either, but it's not worth my time to screw with a work-around.
edit: no, wait... you want the distance between the mounting screw holes.
One holds the LNB in place on the I, and the other holds the I onto the arm.
That is the total answer!