Found an old post in another forum discussing placement of a disc tab on a fork blade. One person mentioned that he feels mounting the disc tab, IS or post mount, not flat mount, offset to the side of the fork blade is preferred over mounting it on the centerline of the blade. Something to do with reducing a stress point where the tab ends on the blade.
Anyone able to explain the logic behind this? A riser will occur wherever the tab ends regardless if it is tangent to the tube or right on the center line of the tube. How far off am I?

I do understand the goal of "centering" forces by joining stuff on centerlines. But when blades go from a 100mm dropout spacing to a crown that is narrower it is hard to keep the caliper mounting bracket on the center of the blades. I'll bend a Willets style bracket to better track along the blade but it isn't on center. I can't say that I see a lot of blade failures that would suggest this is a big issue.

I don't think this really maters. Unless one is REALLY pushing the amount of materials used and thus the structural strength well beyond what is pretty common for builders to work with. Andy.

Yes you might have a riser wherever the tab ends, so the question is, is that in a part of the blade that's under a lot of stress from riding? His thinking might be that the sides of the blade aren't under as much stress as the centreline since that's where it's bending as you crash over potholes etc. So spread the work across the fork. But when you're braking the tab is being compressed into the fork anyway. It's usually tension that breaks or fatigues metal. You could consider putting it off to the side I guess though if there are no downsides to that. If it means you need to make a spacer to attach the caliper I wouldn't consider that preferable.

The tab goes where the geometry of the fork blade dictates. Unless the fork blade runs perfectly plumb from the crown to the dropout, there will always be some offset between the top of the tab and the bottom. If you have to bend the tab to get it to land on the blade in your preferred location, you are just adding an angular force to the attachment point. and a higher chance that your tab is not in the same plane as the rotor.

As Andrew said, as long as you are not pushing the limits with the blade thickness, the tab placement doesn't really matter. I don't have any better pictures of the brake tabs, but the tabs on this fork are just individual tabs welded onto the .049" wall 3/4" tube. That fork has seen hard use on my MTB with a 180mm rotor for over 8 years now.