Richard,
We don't have a running tack
line on our asym arrangement. The tack of the sail has a fixed pendant, about 30-36 inches in length, that
shackles to the sprit just forward of the attachment point for the headstay (on our boat, the attachment point for the headstay has three holes in it, with the headstay affixed through the aft-most). The tack sort of "floats" out above the pulpit, shifting from side to side when we jibe the sail. We adjust luff tension via the spin halyard.
Much better to have a running tackline like yours. I assume you have that block all the way forward at the headstay fitting, more or less in the same location that we
shackle our tack pendant. [You mentioned the "forestay" fitting in your note above, which to me usually refers to the deckplate on the foredeck well aft of the headstay -- which wouldn't be a good place to run the tackline.] To reduce fouling/chaffing, you might consider a lead arrangement similar to the fairleads and blocks used for the roller
furling control
line. I don't have any specific suggestion beyond that -- it's probably a case of trial and error.
Maybe Larry or Adam has a suggestion....