Thanks Puff
After Dan's comments about the length of the tang possibly being longer than usual I decided to fully remove the blade. The Tang is 4.5" (11.5cms) long , the blade is 19.5" (49.5 cms) this gives a ratio of 1:4.3. (previously I used a magnet to check length of tang whilst still in the handle and it was only 3mm out....so a magnet 'test' seems quite accurate for those that want to measure their own tangs)
Assuming that longer blades would require a longer tang a 30" blade would have a tang of 7" (well 6.976" if you wanted to be accurate
)
Does that seem about right?