OK, so I did a little digging, and decided probably not a good idea to use this Blue Hawk stuff. The double braid polyester yacht chord that most people seem to use for this has a tensile strength of over 2,000 lbs. Much more handy if, for any reason, the horse were to set back against the halter. That's a pretty huge difference, so I decided to order some from ebay. This seller Rope, Double braid polyester rope items in Columbia Basin Knot Company LLC store on eBay!
has a good selection. It's a bit more expensive than the rope I bought last night, but far less likely to break I think.