I think you're asking how far (in it's upwards/downwards leeway) does it cram into the lower grommet? It's really tricky, but best explained here:http://business.baylor.edu/Richard_Easley/autofaqs/tseals.htm
I've done it several ways and mine still "seeps" a bit.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)