I have never built a yagi using Hygain elements, but I have built
some 6 element OWA's like you want to build. The effect of
the clamps will be larger than "4th digit", probably a couple of
inches at 28 MHz. But you can ssume to a very good approximation the clamp
will just shift the resonant freq of those elements by a certain amount.

When I built my OWA's I used DX engineering insulated element mounts. With
these, the SWR curve of the real antenna matched the tapered NEC2 model
perfectly. I didn't even need to adjust the length of the driven element.

If you were using a Hygain element for only the driven element, I would
just allow for a few inches of adjustment + or - (hose clamp on last segment),
and just adjust the tip length of the final antenna to get the correct
SWR curve. If you want to use Hygain elements for Dr and 1st Dir, just do
the same, but adjust the lengths of both Dr and 1stDir by the same amount.
In the OWA design changing the length of the Dr and 1st Dir is mostly
going to affect the matching of the antenna.

Tor
N4OGW