The bow is 68" ntn I did the standard minus 3" for string length. That 65" string only gives me a 5" brace. How much shorter does my string need to be to get me in the ballpark to tune it?

Measure the string on the bow (under load).  Depending on the material, it can stretch a lot (if you didn't pre-stretch).  Probably need to go a good 1.5" shorter on the next string.

Under load it gets to about 65 1/2". Should I go for a 64" string. These are prestretched.

I make my own strings and go 3" shorter then the bows ntn length for recurves and longbows.I like to run the brace on my 68" Hill's around 6.25" to 6.5",5" is alittle low and probably a wrist slapper.What kind of string material are you using?Dacron can stretch a bit.Did you measure the strings length to be sure it is still 65"?A 65" string should have put you in the 6" to 6.5" range.Did you make up the string,if so move to the next pin and make a new string alittle shorter.

I didn't make the string but I've measured it over and over because it's not adding up. I've put a ton of twist in this string and it's just not going to make it to even 5 1/2" brace.

How many strands in it?  If you get another from the same place, probably 1.5" shorter.  At least an inch shorter.  I'm with you--something isn't adding up.  Maybe the bow is mis-marked.  

Do you have an old string that puts it at proper brace?  If you do, put the old string back on and measure it with the bow strung.  That should answer a question or two.

I just put a 64" on it puts it at 6 1/4". So it took a string 4" shorter than ntn. This bow was made by Maurice Cash and does have some backset. I've never seen an actual Hill bow. Do they have backset? Maybe that's why it needed a little shorter string?

Bows marked "AMO" are supposed to get a string 3" shorter than the AMO bow length, regardless of what it physically measures.  Unfortunately lots of bowyers don't follow AMO specs, which just makes for confusion.

Actual Hill bows come in several different designs.

^ LBR nailed it. All of the "rules" are just starting points, and the motto "your milage may vary" is a gold standard. Its ok, send it back and get the correct length.

What material are you using??  I have a 68" HH and 65" was perfect for it to get 6 3/8" BH but also still have some leeway to go lower or higher.  I wouldn't go any shorter than 64" on a properly made low stretch material.  Forget B50, hate the stuff.  FF material with not too many twists in the string will reduce hand bump and make the bow more enjoyable.

Oh and a 68" Hill can be run as low as 5.5" BH, Howard was known for using low BH's, and those guys wore arm guards for a reason.  Hills are a straight limb low tech bow not a hi-po r/d rig.  Experiment and see what works for you.  I don't like my Hill brace above 6.5" on a 68 " bow I feel it ruins the draw feel and slows the bow down.

I use b50 I just prefer it. Also there are no overlays on this bow so loops are padded. 64#@28"

Im with nineworlds (as usual) on that, the front tip of my 5" feathers goes right to the back of my shelf. that puts me right at 6.5". I could maybe tune it to a different brace, but why? Low brace is how they're meant to be run.