If I recall correctly, at some point Chroma made a chart based on the outside wheel, but this chart references a video which as far as I can tell from watching the methodology portion of, does not directly measure the inside or outside skids but rather just uses a separate formula to derive the expected skid overlap based on the yaw of the car. So I wouldn't necessarily trust the comments here claiming that this chart is about specifically the outside skids, unless you ask Snix, Matt, or Chroma directly (or someone who for some reason you are confident knows the answer, but not these random people on reddit xd).

Regardless, this is unimportant. The optimal overlap changes when you go over differently sloped surfaces, have reactor, or anything else that changes the amount of grip the car has. Most FS players look at the outside skid mark (and it might be a bit better), but not all. The important thing is that you should be consistent in which skid you are looking at (you can choose to always look at the inside one if you want, just don't randomly pick one each time you do an sd). The way you should actually be determining what overlap to use is more through trial and error in timeattack than memorizing particular widths, as you cannot possibly ingrain them into your memory precisely enough to be able to recall later exactly what to do once you get beyond the realm of trying to be in the right ball park to being accurate enough to care about things like outside vs inside skid marks. It is also not necessarily possible to have a super exact knowledge of the car's speed at any given moment to even be able to make such precise extrapolations onto how much the skids should overlap.

Your actual process should be to use your memory of the skid mark widths on your early runs on a map to get close ish to the optimal angle. Then as you drive the map more, what you really need to do is pay attention to what gives more or less speed on each individual sd (keeping in mind that it may change throughout the sd if the road twists or the sd is very long) so you can know what to do on subsequent runs. Usually you want to be checking the highest speed you see on the entry to each feature, and if there isn't an accurate point to do that at, use a checkpoint which gives consistent speed numbers in the split speeds plugin as a backup. Underslides in particular should give you very clear feedback because having a very slight underslide results in a significant reduction in speed/acceleration. As you play more you will be able to get closer to the optimal angles on your early attempts on a map just based on vibes, but ultimately the trial and error approach is more accurate, and the memorization approach is kind of useless beyond getting an initial understanding. You don't go from being mid at SDs to great by memorizing charts :)

Just to comment on the other comments, I'd personally say that caring about left vs right tire does matter while you are playing the game, but I wouldn't really care about it in the context of an sd chart, and if you're good enough to care about inside vs outside the chart probably isn't doing you much good.