When bottling a whole batch, it's easy. You know how much beer you have and it's easy to measure out the sugar.
Lately I've been trying to keg half my batch and bottle the rest. This is not straightforward because you have to measure out the beer first. I typically cold crash/dry hop in kegs. I could rack the beer from the keg to a marked bucket, and put the sugar in that, but I'd rather just bottle out of the keg.
If you assume bottles are all 12 ounces then you can calculate how much sugar to put in, but the quantities are small, like 2g per bottle. I have made syrup before and used a syringe to measure into the bottles, but that's pretty slow. I've used teaspoons, but that's a pretty crude measure and it's hard to get it in the bottle. I thought about buying some of those carb tabs, but I've heard bad things about them causing flavors. I'm thinking about priming the whole batch and just letting what's in the keg self-prime, but I'd have to let it warm up for a couple weeks and I don't think it would work with that much headspace.