[Pressfit]

If at all possible, always go with a scale longer than your travel. Because if you use a shorter scale you are essentually reducing the capacity of the machine.
You may never actually use the last 5 inches of that lathe travel for a job, BUT if you or someone else forgets and quickly wheels the carriage down to the end, you run the risk of busting your DRO scale off into the chipbed.
I know: I couldn't begin to count the times Iv'e had to repair them or just throw the scales in the trash.

It's pretty much the same with a mill.

Just thought I'd add:

-The best place to mount it for the crosslide on the lathe that I've found is towards the tailstock side. It's safer there from possibly dropping a chunk of stock on top of it or even throwing a chunk of metal out of the chuck onto it. The kits usually come with a hardstop so you can mount it below the reader so someone doesn't slam the tailstock into the glass bar or Reader. Be sure you try to think of everything that can happen to damage it.

-Drill and tap for your mounting bolts in a course threaded bolt style. Fine threads don't work too well in cast iron because of the porosity of it.

-On Mills I usually mount them on the back of the table as once again, they're usually safer ther from damage. Since they are mounted at the ends of the bar, there will be a gap between the bar and the table. Fill that and all gaps with Silicone Rubber caulking so chips don't start getting wedged down into the gap. If chips get in that gap they can actually start wedging the rail in a bow - which of course will affect your accuracy.