Getting proper measurements can be quite tricky, so I wouldn’t be surprised if your measurements are off a bit. No major deal, unless you’re a perfectionist. I assume you’re running a 3/32 chain not 1/8? I would say 1.5mm is perfectly fine. You likely won’t notice any excess wear, and you’re not in danger of throwing the chain, as long as the tension is proper. Happy riding, cheers.

It would be hard to imagine a cog bolted to the disc rotor mount, being within a reasonable chainline with your crank. In fact, it be surprised if it was. I’m also not surprised that your initial measurements were off. Even with callipers it can be real tricky to get an accurate measure. However, if your rotor mounted cog isn’t meant to be used regularly, but as a ‘backup’ or ‘bailout’ option, then I don’t think that chainline will be an issue. I would never recommend that for everyday use of course. If you are running a 3/32 chain, then you have more than enough lateral flex to utilise such a mismatched chainline. But again, that’s if it isn’t used as your normal everyday drivetrain. My general recommendation is anything up to 2mm is perfectly fine. In fact, in general use, it probably won’t be any different from technically perfect. This is a bit different of course, with a 1/8 chain. Particularly a track chain. Anything over 2mm though and I would recommend against it. That’s not to say it’s dangerous, I just wouldn’t recommend it. Others might feel that line sits more at 3 or 4mm. My comfort is at 2mm. My chainline is off by 0.3mm, but I run a 3/32 chain, so it makes exactly zero difference.

my advice is to adjust your drivetrain, so your main cog is as near to perfect as you can achieve, and not worry about the chainline of the rotor mounted cog. It’s pretty hard to derail a single speed setup with proper chain tension. Practically impossible at the numbers you’re looking at.