I've got photos along with the methods & results of my aioli experiments over the weekend, and produced the following base recipe:
1 egg yolk
3-6g garlic (1-2 medium cloves)*
160 mL oil**
1 tsp lemon juice

*I originally had 7-9g, or 2-3 medium cloves; this was way, way too much garlic. I tasted immediately after adding it, and it actually takes a while for the garlic flavor to come through. 1 clove is plenty.
** Extra-Virgin Olive Oil gives you the most flavor, but it's not stable; after a few hours, it will start to break. That's fine if you intend to consume it all immediately, but if you plan on storing/refrigerating it, then you need a mixture of refined oil and EVOO. Thus far, I've tested a 50-50 mix with some success; I'll update this post as I try other ratios.
My first step was to figure out just how much garlic is in a clove.

The largest clove in the bulb was 10g; the smallest ones from the interior are 1g or less. The medium cloves were typically in the 3-4g range. I've read that smaller cloves are more intensely flavored than large ones, so the garlic power is the same, but that doesn't mesh with my experience. However, I have noticed that fresh garlic from the farmer's market is considerably smaller and more powerful than the supermarket varieties, so maybe that's where the idea came from. From the same bulb, though, there doesn't seem to be any difference in strength between individual cloves. Regardless, that will have to wait for a future experiment.
After adding about 50 mL of oil, I had a stable emulsion:

But it started to loosen up around 100 mL, so that's when I added 3/4 teaspoon of lemon juice:

At about 150 mL, it was loosening again, so I added another 1/4 teaspoon of lemon.

It started to loosen again at 160 mL; clearly, I was reaching diminishing returns. Another 1/4 teaspoon of lemon, plus a few drops of oil, and it thickened and stabilized again nicely. I'd have been satisfied stopping here, but SCIENCE! demanded more oil.

Clearly, that was too much; more lemon juice did not help. It took 180 mL of oil to get to the point shown in the photo, but it really started to break at 170 mL. I added more lemon at 170, but it no longer helped. I concluded 160 mL was about the most I could emulsify with 1 egg yolk.
I used refined oil only on my first batch; no sense wasting the good stuff on an emulsion I'm trying to break. I made a 50-50 mixture of oils on my second batch (80mL refined oil, 80mL EVOO), left it out for a few hours, and then put it in the fridge; one day later, it's still stable. The flavor is noticeably better than the 100% refined batch (even before it broke). I'll try the 25% refined/75% EVOO mixture next weekend.
I also tested out how much garlic to add. Using a bulb of garlic from Whole Foods, 1 medium clove (3g) was too little, but it took a second clove (4g) to really give it a decent flavor.

I microplaned a third clove (4g), and added it piecemeal to see how the flavor changed. Around 8g seemed to match the aioli I've had in restaurants, but I actually prefer a little more punch myself, and went with 10g. I thought anything more was overpowering, but your mileage may vary.
ETA:
As noted in the posts below, it takes at least an hour for the garlic flavor to permeate the aioli; when I tried it again a day later, that second clove (followed by the third) was way, way too much garlic.One caveat: as is obvious from the photos, I have neither a mortar & pestle, nor a food processor, so I prepared this with a fork and a bowl. The rough surface of on the mortar and pestle may create better agitation, and a food processor definitely generates more RPMs than I ever could have, so the emulsion might be more stable using those methods (or by someone more experienced than I am at drizzling oil in one hand while beating a fork with the other). For my own level of cookery, though, I won't try anything above 160mL oil per egg. Which suits me fine, because I like that egginess behind the garlic.
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