I've driven quite a bit at 95% and it seems ok for the most part although 97 to 98 feels a tad more correct pending the car choice. I've been driving the 964 in R3E a lot and the grip levels in that sim seem to be quite good with a gradual drop off.
There were several different recommendations over the years as updates came and went. These days I run 98 for public roads, generally. Feels good to me using kunos-style tires. I know @mclarenf1papa calibrates his tires for 100, so as he says, it depends heavily on the car creator.
It also depends on the track. There's a coefficient set in each track's surface files, and the global setting modifies that, unless I completely misunderstand the system.
@CobraCat The reason we can set the track to the level we want is just for that reason, we all have our opinion on what feels right. I always felt that 100, which is maximum grip, is there to cater to people driving with controller or not really sim enthusiast. I never use optimum, 97 to 95 is usually more realistic, to me.
I drive in VR, on a road like LAC, driving over 97 with any other tire than street tires, is not representative to me of what driving IRL is about.
To each his own, that is why we have a choice, set it to what feel right to you in relation to what gear you sim drive with and what your experience of real life driving is.
The actual reason we can set it is that it's there to try to simulate real track conditions, but as I said, it's arbitrary, so it depends on the car and track (e.g. Laguna Seca has lower grip in its files than other Kunos tracks so "green" at laguna =/= "green" at Silverstone). It's just there to give some variety.
Excellent info! Thanx for the replies! Typically I run 97% grip although after watching this video I wondered if I should set it abit higher for LAC and I recalled the post by Aris a while back suggesting that since the updates the grip level should be set in the high 90s. Thanx!