You're going to find that this is a "moving target."

First, let's consider fixed-shape thumbnails. If you choose image bounds of 300x200, Neptune displays them at 80% of that size - 240x160. Why? Two reasons. First, the thumbnails expand when you hover on them. If they were already being shown at their full size, the expansion would drive them into "fuzzy" territory. Second, the skin does this so that if you switch between fixed-shape and justified, the transition isn't so extreme that it requires new image bounds.

But now we get to the fun part - the justified gallery. Make an album. Preview it. Now grab your mouse and make the browser window narrower, then wider again. You will quickly discover that the row height changes. Why? Because it has to. That's the only way it can maintain fully-justified rows of thumbnails. I do some limiting of the row height, to prevent blurring. But I also set the target row height to 75% of the thumbnail image bounds height, also to prevent fuzzy images. But that's just a target, not an exact measurement.

In short, you simply can't let yourself get fixated on the precise measurements involved, because the row height will vary, depending upon the thumbnails and the viewport width. In fact, just changing the sequence of the images will often change the row height! Again, the only way to achieve a justified gallery, without cropping the images, is to let the row height float.

Pick bounds that create an album that looks good to you. Thumbnails too small? Set the bounds a bit larger, like 330x220, or 360x240. That's all you can do, unless you want to use fixed-shape thumbnails.

Oh, yes, that would be a disaster. It would, of course, make every thumbnail precisely 1000x160px, which would produce a strange result, indeed. "Fixed shape" really does mean "fixed shape!" But for justified thumbnails, the wider image bounds are better at handling originals that have more extreme aspect ratios, e.g., something more than 2:1.

Now that Neptune allows fixed shape thumbnails, I've changed the default bounds to 300x200, precisely to avoid that unpleasant outcome. The release notes warn about it, in fact.

I've chosen 300x200 because the 3:2 aspect ratio matches a lot of digital images, so it's a good "most cases" choice.

I've gradually started gravitating to larger thumbnails. As long as you've got a responsive layout, there's no huge penalty to letting them get a bit bigger. And bandwidth isn't as much of a concern as it used to be. In fact, I tend to use "high DPI" thumbnails more often - the image files are larger, but they really do look better.

My own family album uses Pluto (same thumbnail layout as Neptune), justified thumbnails, bounds of 330x220, high DPI, and folder thumbnails at 140% height. But my public "travels" album uses Neptune, justified, 1000x160, non-high DPI, and folder thumbnails at 150% height. But I'm probably going to bump some of that up a notch.