How much hop at the joint in the rim is acceptable?
 

The pinned joint has a little bit of a sharp edge that is radially misaligned by 0.1mm. I've trued them so that the joint / seam is +0.4mm radially (including the 0.1mm sharp edge), the 1st spoke on either side of the seam is approx. +0.25 mm radially, 2nd spoke to either side is approx. +0.15mm radially, and the rest are uniform. I use a folded piece of paper to measure. Is this amount of hop acceptable?

These are 20" (406) rims with 20/28 holes, so probably more difficult to adjust than larger wheels. After spokes are tensioned, an additional 1/4 turn of the nipple increases tension by 1 tone (when plucking), and my cheap tension meter says it's an additional 15 kgf.

The 2 drive side spokes nearest to the seam are already 1 tone higher (when plucking) than the next highest drive side spokes, and turning the nipple on these 2 spokes make them extremely stiff compared to the others, so I thought leaving the hop around the seam would be better than over tensioning these 2 spokes.

Lateral deviation are all within +/-0.05mm, except +0.1mm at the seam towards non-drive side, which I allow, otherwise 1 of the drive side spokes next to the seam would have to be tensioned further, as it is already the highest tensioned. And the non-drive side spoke at the seam is already the lowest tensioned, so I prefer not to lower it anymore.

For me, a .4mm radially out-of-true rim would be acceptable, even on a road bike. I'm supposing that your 20-inch rim build is for an off-road use? The tires you put on the rims would certainly never be within that same tolerance, so I can't see how you'd notice that small of an amount.

Few tires are radially perfect and no roads are perfectly smooth, so I would say that your 0.4mm radial hop ought to be imperceptible.

I'm surprised that you are finding it necessary to tighten the spokes adjacent to the rim seam. I usually find the seam to be a little bit flat, requiring less spoke tension to achieve a perfectly round rim.

I've seen it lots of times. The seam, especially if pinned, is a discontinuity in the hoop, and stress will tend to concentrate there as the entire rim is compressed. I have often had to add a little extra tension there to counteract this.
As far as 0.4mm radial hop is concerned, that's certainly acceptable. That's probably less than the manufacturer's own tolerances.

'Been my experience too. I've built about a dozen wheels with pinned seams and I don't remember one that did not have a high spot at that location.

Dean

If you mount a tire on the rim and put it in a frame and watch it rotate, you will probably find that variations in mounted tire roundness are due mainly to imperfections in the tire and not in the rim.

Have had many rims that didn't want to be perfectly round when evenly tensioned and laterally true. Spent a lot of time trying to get them round and never found a method to do it. Best I can figure, the way to get it as round as possible is to work on roundness first and then work on true and dish and when it is round an true, work on getting reasonably uniform tension. But, the problem seems to be inherent to the rims and in the end, slight errors of roundness seem more tolerable than uneven tension or lack of true.

+1 to above - I try to build my wheels as true as possible, but if there's no way to "smooth" the joint 0.4mm is OK for me.

You can use a file to smooth it out if you want, but you don't need to.

The wider the tire you intend to use, the less noticeable this will be.
I wouldn't worry about it as long as (a) the spoke tension is in a stable, sustainable spot, and (b) the tire bead seats correctly and the dicontinuity doesn't put any sharp pressure on the tire. If it does, you could use a file as @noglider suggests.