Hey gang. Haven't been on here in a while, but I'm still out here, shooting and hunting. Having a little trouble I could use some guidance on.
I've been shooting nothing but Hill-style longbows for years, but recently bought a Damon/Howatt Mamba, used but nice. It's marked 55# @ 28, 58" bow. I came with a new string which measures 54", so by the old standard that should be about right. Problem is, twist it up as I might, I can't get the brace height anywhere close to D/H's recommended 7 1/4" to 8". I'm barely at 6". Do I need a 1/2" shorter string?

That should be the right size, but the proof is in the stringing.  Did you measure the string to make sure it's 54 inches? If it's a flemish string, check to see if one of the braided ends might be slipping?

If you can't get the bow to brace where you want it with 2 twists per inch or less (I prefer one), you likely do need a shorter string.

 

Thanks. I will string again and count my twists and see how it looks. I typically like my bows at the lower end of the brace height range - seems to shoot better with less stack at my somewhat longer than average draw length (~29"), but this seemed way low.

If my situation, I would start tying overhand knots in your string until the brace height is where I want it, then measure the string length, and make or buy a string that length. 

I think stevem has the right idea.  Don't just buy a string 1/2" shorter than the original one, because the original one could have been mismarked.  Instead, shorten the one you have until it fits, and then order a string of that length.  When you get it, measure it to make sure it is the length you ordered.

A poorly made Dacron string will stretch a significant amount. That might be what's going on.

I never go by the rule of thumb that it should be 3" or 4" shorter.  More often than not I end up having to almost totally untwist the string or twist it up so much it starts kinking. I will usually take my tillering string and keep tying a new timber hitch (trial and error) until I get the bow right at the brace height I want. I then take that string and pull it taut between two pegs and measure it from end of loop to end of loop - send that measurement off to my string guy and viola'. I actually subtract a 1/4-1/2" from the measurement I get, and when I receive the string I give it a good stretch with a contraption I made using a ratchet strap. This gets me real close and still gives me a fair amount of twists up or down for tweaking the BH. ]
Make sure you let your string maker know the length you provide them is the "actual" string length from end to end and not the AMO length.

Once I know the string length that works best for a particular bow, I record that information on my smart phone in Notes.  Next time I need a string I just refer back to it.