These Machine Heads are awesome

[MEhelper]

That site is ok if you know the string density, but that depends on the way the string is made, so you're pretty much forced to guess (unless you can very carefully weigh the string and measure its volume!)

Daddario have a site with exact tensions for many of their strings:
http://www.daddario.com/DAstringtensionguide.Page
It's presumably accurate for their strings, but I suspect it would be a useful rule of thumb for most other brands too.

He has a calculator for that as well, check the density link, its not too hard to measure the length, diameter and weight and he even offers some stndard string material densitys. Its not going to be 100% but it will certainly tell you if your going to have too much tension or not.

[JayGee]

A solution in search of a problem...

Hmmm....

I'm wondering if they might open up some possibilities for compact 12 string (or mandolin I suppose) headstock layouts.

Maybe combining rear mounting with front mounting in a staggered layout. Or possibly a Rickenbacker style setup?