I've only tried AM-2201 and JWH-250. While I do have the possibility to measure out 100ug with a satisfying precision of 30ug (windows closed, nobody move a muscle!), I never felt the need to do that and eyeballed it from the start. I've had various batches and only once did I really go over the top, but that might've been related to an unknown substance passing from my ass into my bloodstream at the same time.
JWH-250 imho sucked, I barely felt shit (only had one batch from the same supplier as the best AM batch I had). I've been smoking pot for 12 years straight, at nighttime, but my tolerance is pretty low to actual pot. 100mg weed will get me pretty blasted every night!!
AM-2201 was pretty fascinating, but I could never replace pot with it and since I don't just change my habit and ended up not using it at all anymore. After 2 days I ingested 50mg (!) along with a softdrink and told a friend to be avilable in case of emergency (he was really pissed, but I've never really or got into any sticky situation in years of excessive drug use and was mostly joking). I dropped off my daughter at home, headed for the train station where I started drink 500ml coke with the stuff + 50mg AM2201 solution. Surprisingly it took less than 10 minutes to notice my face being frozen into big fat warm headed smile. It felt pretty trippy and intense, needless to say I was a bit surprised by the sudden onset. I managed to get home fine and followed my routine.
Now I wouldn't recommend anyone to eyeball, but if you feel you have to, just start with the smallest amount you can still see and sprinkle them into a cigarette or whatever. You might even feel some effects, but if you don't, then just double the dose and so on... Liquid dosing would probably be safer, like 10mg per ml in acetone applied to a cigarette with one of them skinny 1ml syringes. But tbh I've never felt the need. I'm a stoner with a very low pot tolerance and AM made it shoot through the roof within a week or nightly smoking. The cross-tolerance seemed limited only to some aspects of the effects, probably won't surprise anyone considering it's just one active substance compared to 60 in cannabis.
I do have a few friends who have made bad experiences with the stuff, but after all it's just a cannabinoid overdose, serves them right. I had a few as well, but nothing that comes close to what people describe (as in full blown trip). Not saying that can't happen or the stuff isn't severely impressive, but it's still in the same league as cannabis, without the richness of the effects.
One more thing to note would be the short duration which would reliably wake me up at 3am like some sort of early rebound effect I guess. Still (or maybe because of that) I felt like a zombie after 3 weeks (of both AM and cannabis use) and pretty much went back to smoking cannabis alone. While the doses seem miniscule, I could easily see myself smoke half a gram a day and that can't be healthy, toluene ring cancer crap or not, half a gram of chemical in my lungs cant be that healthy.