you can buy a double grommet kit from Hobie, it comes with everything you need and you can just do it your self. I myself if the tramp is in good shape would do the back and the middle to maximize the life. As for the lengths of the rope you can look that up in the Hobie Catalog, but you don't need that much.
Which brings up the question:
If Hobie tramps perform much better longterm (in some way*) with more grommets, why doesn't Hobie just offer them them that way? At least as an option?* Certainly not resiliency or longevity (see below)
A related question:
Is there any (H16,H14) racing-related stipulation about tramp grommets, lacing, etc. ?
What is the tramp lacing like on the high performance cats like Tiger, Wildcat, etc. ?
I am adding more grommets to the tramps of my Turbo 14s... but that is because they are already 20+ years old and still in fairly usable shape- and
I got a deal on ss grommets and tooling.