My smoothing plane leaves a track line down the board every pass, is that a camber problem, and how much camber is right?

Yes, that is exactly a camber problem, and the reason the numbers vary so much is that they are answers for three different planes doing three different jobs. Sort by job and it becomes simple:

Jack, for hogging material off fast. Heavy camber - a visibly curved edge, something in the region of a few millimetres of curve across the width. You are taking a thick shaving from the middle of the iron and the corners are nowhere near the work. Nobody expects the surface to be finished.

Jointer, for making an edge straight and square. Very slight camber, almost imperceptible. You want a flat cut, but you also want to be able to steer - with a hair of camber you can correct an out-of-square edge by shifting the plane sideways, which is genuinely the main technique for jointing. Dead straight takes that away.

Smoother, which is your problem. Slight camber, more than the jointer and far less than the jack. Enough that the corners never touch and the shaving fades out at its edges instead of ending in a step. This is what removes your track lines.

The principle underneath all three: camber is how much of the iron's width is cutting. More camber, narrower and thicker cut. Less camber, wider and finer. The tracks appear when the corner is cutting at full depth and then stops abruptly, which is a step in the surface - and once you see it that way the whole thing is obvious.

For a smoother you do not need a jig or a number. Finger pressure gets you there and it is what most people actually do.

Sharpen normally, then on your finishing stone take a few strokes with extra pressure on one corner of the iron, then the same on the other, then a few in the middle. More strokes at the corners removes slightly more metal there, and that is your camber. Ten extra strokes each side is a good starting point.

Test it before you go further: hold the iron up against a straight edge and look at the light. You want a barely visible crescent. If you can obviously see a curve, that is too much for a smoother.

The better test is the shaving. A correctly cambered smoother produces a shaving thinner at the edges than in the middle, and it will be narrower than the iron. If your shaving is full width with square edges, you still have no camber. If the shaving is much narrower than the iron, you have overdone it and you are now taking many more passes than you need.

If you use a honing guide, the same trick works: pressure at the corners while the guide holds the angle. Some guides have a wheel narrow enough to let you rock slightly, which does it faster.

Easing the corners alone is not quite the same thing, but for a first attempt it removes most of the track lines and it is very hard to get wrong.

Worth saying that the corner tracks can also come from a perfectly straight iron set too deep. Before recambering, back the depth off until it is taking a shaving you can read newsprint through and see whether the tracks survive.

If they do, it is camber. If they do not, it was depth, and you have avoided regrinding an iron that was fine.

Finger pressure on the finishing stone, five strokes each side, check with a straightedge. That is the entire technique.

Sanding out what the plane should have finished is the most demoralising step in woodworking.

The numbers people quote in thousandths are real but they are usually answering a different question than the one being asked, which is why they are confusing.

What actually matters is the relationship between camber and depth of cut. If your camber is deeper than your shaving is thick, the corners cannot touch and you get no tracks. If your shaving is thicker than the camber, they will. So a very fine smoother needs very little camber, and the same iron in a plane set for a thicker cut would need more.

Which is why one specific number for a smoother is not really available, it depends on how fine you are working. What people mean when they say a couple of thousandths is roughly "more than a fine shaving and less than you can obviously see".

The practical consequence, and the reason I would not chase a figure: if you get tracks, add a little more camber or take a finer shaving. Those are the two knobs and either fixes it. Adjust and test rather than measuring.

One thing worth knowing that does need a number, though: whatever camber you put on, it has to be symmetrical, or the plane will cut deeper on one side and you will chase a taper across the board forever. Checking against a straight edge under a light is the whole quality control step and it takes five seconds.