First off, I have a simple table that shows the probability of making it into assault at certain distances.
So the last bit I have here is for units with the "fleet" special rule, and is actually from 40kmath.com. This is a great article, and this guy did some serious number crunching to figure this out so thank you to him!
Based on this chart, rolling everything that is 3 or less (assuming you didn't make the roll you need) will generally get you the best chance of making the assault. The only exceptions are when you need to assault less than 5 inches, or more than 10 inches.
For those of you who use units with the fleet rule, this chart will be very nice to have with you (The guy who made it has a cheat sheet here that you can print out and take to your games). He also has another chart that shows the same thing for moving through difficult terrain.
Well i hope that someone out there found this useful/interesting. Hopefully I can get some modeling/painting time in here soon. School is kinda killing my time right now though (only one more year until I'm done with school forever!) Thanks for reading!