As always Jens, you pose fascinating questions, as only a master of the esoterica of Indian arms with many decades of study could!
It seems this topic has come up a number of times over the years, and it seems like it was years ago when we were poring through A.L. Basham's great work.
It seems the barley corn was a key element in standards of measure in a number of regions and cultures (it seems it remains a measure in U.K. shoe sizes if the note I saw on line is correct).
In study on the Khevsur's of the remote Caucasian regions of Georgia, their tradition of dueling typically did not result in notable wounds beyond bruises, however if a man was wounded, he was entitled to compensation.
This was determined by measuring each wound using BARLEY CORNS.
("Seven League Boots", R.Halliburton, 1935).
As you note, the key is 'how many barley corns determine the finger width?'.
Using a standard of measure which could obviously vary depending on the item being used ( an arms length; a pace or foot length etc) would result in certain disparities of course,
I think most of these would be 'rule of thumb'
sorry.
The length of the shamshir by using geometric methods seem like it would be by the measure from root to blade point in straight line, rather like TV sets are measured by the straight diagonal corner to corner on the screen.
Like most things, determinates are relative and it seems subjective.
The best blades (or 'good' blades) are determined by length? It would depend on by whose standards. A 'good' cavalry blade is better if longer for its reach from horseback.....but then Rajputs usually dismounted to fight, so then would a shorter sword be better...for the close quarters melee?
A lot of relativity, subjectivity and interpretation as far as understanding what these things meant in their times as described by contemporaries, but opens intriguing doors to understanding them better.