Q: I have a pretty basic question. As an upright 6'-5" tall man, I should be considering blades with a tang steepness of 30 degrees plus?
A: Peter Vido asked me to address your question, as I am 6'-8" tall and have faced the same issue.
In general, you could say that the taller you are, the steeper a tang you need, but the tang steepness you need depends mainly on the snath you will be using.
For example, the first scythe I got was from Johnny's Selected Seeds. The blade's tang angle was about 25 degrees. The snath was woefully too small for me. I modified it as shown in a photograph on this webpage:
The snath modification now allowed me to mow with a straight back. With this modified snath, 25 degrees was the right tang angle for someone of my height (but the ergonomics of the snath were far from ideal).
I later made a one-grip snath for the same 25 degree blade. I followed the instructions listed here:
To make the one-grip snath work for me with a 25 degree blade, I had to use a piece of wildwood for the shaft (instead of sawn lumber) which was specially selected to have a curved bottom. (The wood was from the prunings of an ash tree in my backyard.) I sawed the curved bottom at just the right angle to accommodate my height and the 25 degree tang angle. With this snath, a 25 degree tang is the correct one for me, and it works remarkably better for me than the same blade with the modified Johnny's snath. (For example, with the Johnny's snath, I can't get a full swing with enough control to cut well at the end of the stroke.)
Without a curve at the bottom of the one-grip snath, in other words with a straight shaft, the tang angle would have to be much steeper. Exactly how steep can be calculated by the method quoted from the following webpage:
"To estimate the required tang angle [for a straight-shafted one-grip snath] for anyone, regardless of height, a scale drawing can be made on graph paper, or a bit of trigonometry can be applied. Assuming the person is right-handed, measure the the height of the right hand in the mowing stance (usually with the right arm almost completely straight), and divide this number by the theoretical distance between right grip and end of snath (following the instructions for making the one-grip snath), and find the arcsine (or "inverse sine") for this quotient. The result is the approximate tang angle required for both the one-grip snath and the Oregon snath."
I wanted to make a straight one-grip snath and this method resulted a tang angle of about 50 degrees! Luckily for me, Peter happened to have some suitable blades in stock (he has many blades not listed in the catalogue).
I later wanted to try these same blades with a two-grip snath (homemade from straight lumber), so I designed the "Oregon Snath", as shown in the above link.
If I had a two-grip snath with curves in the right places, I could use a less-steep tang angle. How much less of an angle depends on the specific snath design (the location and amount of curvature, the grip arrangement, etc.)
The adjustable two-grip snaths that are available from various sources are only adjustable for a certain range of heights, and this doesn't really address the blade fit. Each adjustment of the snath might change the angle the snath makes with the ground, theoretically requiring a different tang angle.
For a given user/snath combination there is an optimal tang angle. (The desired use for the scythe can be reflected in the snath design and influence the optimal tang angle.) The available blades may not match this optimal tang angle. There is some leeway possible since the user can adjust his/her stance and arm positioning a bit to make a blade work, up to a certain limit. Beyond this, a wooden wedge can be used between the tang and the snath, to simulate a decrease in tang angle, up to about 5 degrees less than the actual tang angle. Modifications are sometimes made to the bottom of the snath, tapering (cutting a wedge off) the snath to simulate an increase in tang angle.
The use of wedges and tapering to affect the lay of the blade is explained in this webpage:
A recap of the last couple points: A snath can be fitted to the user. The resulting snath will need a blade with a certain tang angle. The curvature of the snath affects the required tang angle of the blade. A blade with a tang angle that is too high for the user/snath combo (up to about 5 degrees too high) can still be used with a wedge. A blade with a tang angle that is too low (up to a couple degrees too low) can be used if the snath end is tapered.
With the Oregon snath, I took a different approach by starting with a given blade. The Oregon snath is then made to fit the user (and blade) without the use of wedges or tapers.
The blade/snath/user each have an influence on the other two factors. For example, you cannot determine the tang angle for a given height user without considering the specific snath.
The maker of the snath should have this all figured out, but this isn't always the case. I think that there may be some fudging done, such as one or two tang angles used for a range of users, instead of having a wider range of tang angles available to properly fit the range of users.
So I hope that you now understand how a 25-degree blade and a 50-degree blade can both fit the same tall person, and how the answer to your basic question depends on the specific snath.