had a quick look on the Border website but couldn't find anything about what the recommended string material/count is for the Border Hex 6.6 limbs (long limbs on a 25" riser) - what should I be looking for?
strand count is going to depend on material and the nock fitting...
Dynaflight 10, for example uses a strand count more like dacron. 8190 on the other hand, you need something like 26 strands to fit a size 2 nock. 8125 and Fastflite tend to fall somewhere between those extremes. The serving material you use also matters; a thicker serving means fewer strands, generally...
When I got my Hex 6.6s back in January, Ann advised me that they recommend Fast Flyte Plus as "it's kindest to the limbs". So I've gone with it and have found that 16 strands of orange FF+ with 0.018 Angel Majesty centre serving give me a decent fit for Beiter #1 nocks.
This is the first time I've used FF+ and have no idea if it has much colour variation, for example with 8190 (before I got the Borders) I found that I needed either 20 strands of white or 16 of red! (which was much more heavily waxed) to give the same nock fit.
This document assumes that the string material is 8125 or 8125G, the serving material in Angel Majesty, and the end loops are Asian style (i.e. there is no overlap of the serving material at the end and loop servings). If you make a string with serving overlap at the ends, add 1/2" to the overall length of the string. Please note that you will need 1" longer end-serving length for HEX limbs. Nock "fit" is defined by my personal preference for a very light nock clip onto the string. Most people, I've discovered, prefer a tighter fit for which you would go up one diameter size in the serving material.
BCY 8125 is lightly blended Dyneema, so will be slightly less elastic than a pure Dyneema. 8125 thus stops the limbs a bit faster when loosed, and gives a nice snappy sound that most people seem to like. I've recently moved to Brownell Astro Flight which is a pure Dyneema and thus slightly more elastic than 8125. There is a bit more limb bounce back on the loose, and the sound is a bit more of a "boing" than a "whack". But I think the slight increase in elasticity will help preserve the limbs. Performance-wise, arrow on target, I cannot tell the difference between 8125 and Astro Flight.