There is no "51% rule" per se. There is the requirement that the majority of the construction be "amateur-built". My DAR explained it this way: the "majority amateur-built" criteria applies primarily to the kit's design, not to how it was fabricated. It is the kit manufacturer who has to document to the FAA that their design requires that someone else must perform the majority of the tasks necessary to produce an airworthy aircraft. So there is quite a bit of latitude. You can have it professionally painted, You may pay someone to fabricate and install the interior. You really should have a professional install your G-8000 (is that what they're up to now?) avionics and ejection seat. Such "assistance" does not count against you, presumably because you (the amateur) "supervised" the task.

The tasks guidelines can be, and are frequently, abused. In theory, if you drill, countersink, drive and buck a rivet, you have acquired the knowledge and experience necessary to perform the "rivet task"; you can let someone with more skill do the rest. To my way of thinking, that subverts the entire philosophy of amateur-building, but I know it happens. For certain very popular kits, there are companies that will take delivery of your crates and, with a bare minimum of participation by you, build and document it for you. This skates on the razor's edge of the E-AB intent, but for a sizeable additional expense, you are assured an excellent aircraft.

For me, it was a matter of personal enjoyment and pride that I did it all myself.