I just got my new FF string from one of the sponsors.  It's a work of art.  I measured my existing string (68" HHA longbow) and it was around 64-1/8"-64-1/4" so I told the maker to go with 64-1/4".  My existing string is the stock, non-FF, string.  I put the new string on and my brace height was around 7-1/2".  I untwisted the string as much as I could without it no longer having any twist.  I let it sit for a few hours and my brace height is around 6-7/8".  My existing string is braced around 6-3/8" which is typically recommend for a 68" HIll style bow.  My fear is I ordered a string that is a little short.  Out of curiosity I took the new string off and laid it next to my existing string.  My existing string is actually nearly an 1" shorter.  Now I am scratching my head.  They only answer I can come up with is that my non-FF string has some spring to it so it stretches at the point it is braced.  I have not shot the new string, if it shoots good with the higher brace fine but my guess is I will need a longer string  to get the BH down.  Just not sure what length I would need since my non-FF appears to be shorter.

I need to revise some of my above info as I just remeasured everything and I was incorrect that my existing non-FF string is almost an inch shorter. I just remeasured everything and here is what I got:
Stock HHA Non-FF string is 64-1/8" and shows a 6-3/8" brace height.
New FF string is 64-1/4" and has a brace height of 6-7/8" and not much twist left to take out.

Dacron strings that I've used stretch out like crazy compared to my comparable length FF strings. It can be irritating, which is one reason I only shoot FF strings on my bows that can handle it.

The original string should be measured for length after it has been on the bow for a while at the preferred brace.  If you measured your string while it was off the bow for a while (especially if it is a flemish twist string), the string length measured can vary due to stretching that occurs while the bow is strung up.

If the FF type string has padded loops they might take a while to settle in.

Yep I use fast flight primarily so I don't have to wear and arm guard. Dacron stretchs to much. Use it on self bows to so I can make brace hight real low.

Even though a 1/2 inch difference is a lot I'd suggest shooting 50 or so arrows to see if the string wants to settle in more.
A lot depends on if the supplier did some pre stretching.
Being down to a couple of twists per inch would be OK.

brace height from bowyer is only a suggestion and starting point,  your tuning will tell you what works with your bow/arrow combo.....I've had to retune or go to stiffer arrows when going to fastflight from dacron so shoot it and see...........

Thanks.  I guess I learned a little bit.  All my string experience has been with HHA strings as in the past I just called them up and said send me another string for my 68" bow.  Never had to measure anything.  I figured since I have been using this string since June it was fully stretched.  There must be a little bit of "spring effect" at play with a Dacron string once it's unbraced and off the bow.  I did shoot the new string and it did well.  It surely seems faster.  My arrows flew nice and straight and hit hard.