I was in exactly your shoes this past November. I built my Rap while eyeballing the links and off I went. It was a real disaster. My experience suggests that you will do yourself a BIG favor to measure them exactly--and then fine tune. It is amazing how much that helps. If your heli isn't set-up well (i.e. basically hovers perfectly with no stick inputs--has the right up/down pitch curves and throttle balance) you will have a very tough time making progress. So...I'd suggest you get a caliper and the link attachments to make that job a snap. I got the Grizzly 6 inch dial caliper--has metric on the ruler and is cheap.(6" dial caliper ) Then I got the pushrod measuring tool from Ricks. (pushrod measuring tool )
These things are my most frequently used tools and for little things beyond the links. (And you'll be doing links often enough with your initial re-builds, etc.) For example, to get the tail right, the various websites talk about a 4.5 mm gap between the casing and the slider. The caliper is handy and quick.
Finally, the next big set-up thing I got hit with was loosening the links. When assembled, the darn links are very tight--so the heli controls are tight. That translates to the heli not responding to little inputs (links stick) and the heli only responds when you move the controls more than a little--so the heli is unstable. The JR ball link sizing tool is a good addition, though you can also loosen them by pinching them with pliers. (An experienced hand will help here.)
AND when you find help--they'll actually be able to do something since the heli will be in good shape to tune-up. Finding a mentor is probably the most important thing of all--but they're not going to be very psyched if you don't do your part.
PS, my mentors include Dragon2115 on this forum, Jeremy Waltzer, and Howard Moftich--who have really been the difference. If you find folks like them, you're going to have a good time of it.