I agree about just finding the lowest angle that holds for the knife and its intended use, it's generally quicker to just shoot for a low angle ( minus 30 degrees inclusive ) and then go up from there until it holds.
To keep track of what angle I sharpened a knife at, I just use a stack of pennies... Most knives I just remember whether I used a stack of 3, 4 or 5 pennies. I find my muscle memory and visual memory isn't great so I like having a static reference. I just keep the stack on one corner of the benchstone and check that I'm still holding that angle ever-so-often by resting the very edge of the spine on the edge of the pennies, being mindful to rest it along the same portion of the spine from tip-to-tang--I usually just match it parallel to brand or steel markings or the ricasso so I don't have to remember. This is just when establishing a bevel, from then on out I just match it when sharpening and touching up by visual or tactile confirmation.
I haven't found knowing the exactly angle useful, but for the sake of communication measuring an angle in units of degree rather than stacks of pennies seems to be more traditional
So to measure that I just take the height of the stack, and the width of the blade ( i.e. from spine to cutting edge ) and then use some trigonometry to find the angle the blade will form in respect to the hone's surface. The blade basically forms the hypotenuse, and the hone's surface the adjacent so you can just do asin(stack-height/blade-width) to get the angle. Just remember to account for half the spine's thickness into the stack's height, assuming you have a properly centered V ground blade--the center of the spine being the centerline the edge will be on.
So in other words... You have a blade that is 1/8" thick at the spine, is 1" wide and you decided it will hold its edge with 3 pennies... A U.S. penny is .061" thick so you have a stack height of .183", but don't forget to consider half the spine thickness so really the blade's centerline will be held closer to .245". So do asin(.245/1) on your calculator while in DEG mode ( or convert from radians ) and you've got your angle of ~14 degrees per side. Of course there's probably a lot of variance in what the actual angle it is but I doubt it matters much and I'm not gonna shell out money for a goniometer and from what I've found it's generally within the ball-park of what general wisdom will tell. In other words, if I measure a particular blade of these dimensions and find it's sharpened to ~28 degrees inclusive then I can expect to make the same observations as someone who uses it at 30 degree inclusive.
I'm not really sure how close to the actual edge angle this kind of method provides. There's another way to do it with trigonometry that involves measuring the thickness behind the edge and the bevel-face's width, but I think that method is even more susceptible to inaccuracies because of how hard it is to get such small measurements. Either way, even if you're off by +/- 5 degrees I think the observations people make about various angles to be well in-line with my own experiences using this method and to me that's all that really counts is being able to properly communicate and relay the information and observation to others.
That's all I really care about angles for, when it gets down to it an edge that's 30 degrees inclusive is probably going to perform nearly identically to one that 33, or one that is 27. There's just really not that much at stake in the numbers themselves, but when it comes to pennies I like them because generally adding just that extra .061" of height bumps the angle up 10 degrees on
most of the blade-widths I use. it gets tricky when I sharpen a blade that's much wider than 1 inch and I find that using only 3 pennies will leave me witch an angle that's MUCH too acute. So for that sake knowing the math and what angles to help for is helpful because I know off-the-bat what height to set the spine at.
I've been meaning to make a set of blocks, probably of plastic of some type since that will be easy to cut and won't swell with water or temperature or anything... But anything I want to make blocks of various heights say .100", .150", .200", .250" and so on up to maybe about a half an inch--maybe I'll sharpen some really wide kitchen knives some day. I know there are some machinist's calibration sets that have little metal blocks in similar measurements but I've never seen them for cheap so I figure just getting a little plastic dowel at home-depot and cutting lengths and lapping them flat would be the way to go.
Though, at the end of all this spiel... I'm still so cheap and lazy I'd rather just use a stack of pennies and a calculator.
