I can't say too much about dosage because my foil vaping technique is abysmal but I'd say if you have experience with small doses in order to get a feel for lighter distance and the speed at which it evaporates, at what angle it'll run down the foil and all that. You can just say fuck it and up drop a large dose onto the foil and vaporizing portions of it every once in a while. I ended up thinking it was better to give it some high/too much heat to have the entire dose get to the desired temp quickly (saves you the time spent mucking trying to get it to sta) Then just follow the the substance up and down the foil with your straw of choice and at varying distances/intervals with your lighter to keep the goo rolling.
HYDRATE , EAT WELL, BE SMART, SLEEP.
If you start hallucinating take it as a warning sign that tells you to stop, you've had too much and are on the path to psychosis or something worse.
I've hallucinated seeing vapor patterns on bright/lit objects, a constant tinnitus-like sound that was similar to sucking on the straw I used, and seeing the straw I used projected onto my vision when close to my eyes/put in my mouth, like when drinking from a bottle of water. Some of my fingertips felt a bit numb for a few days, perhaps due to vasoconstriction and holding the lighter too tighly for too long, wish I/should've owned a lighter that doesn't make you hold something down to make the gas come out/flame stay lit.
Good thing I only had about 1 gram, I don't know when/if I would've stopped if I had any more. I kept redosing trying to perfectly vaporize the entire dose, this was my second time vaping off foil and a bit of etizolam and u-47700 were in the mix so that could've contributed to the low inhibition and urge to get better at it. I used a hard plastic straw so I could bite it to keep it in place, after a while my teeth hurt even though I was on the aforementioned u-47700, the soft straws I had were getting squished by the bruxism and sucked further into my mouth.
After the 55 hour binge I had an 8 hour sleep with the help of some magnesium, clonazolam, chamomile, and melatonin and thankfully all the hallucinations were gone when I woke up.
A few weeks later I salvaged a crumpled up foil ball that ended up having some molten, hardened substance in it. I weighed it, crushed it up, took a test bump (like 2mg) and sure enough it was the strong hexen small my batch had, I have to say I had more of a mood-lift from that than the foil debacle, your mileage may vary.
All in all I'd suggest you set a limit for yourself and stick to it BUT still prepare for the worst in case you start compulsively redosing, like having a hydration/dietary/supplement schedule and for example taking this supplement might be helpful with that.
note: written under the influence (not hexen ;))