Yes you cannot 100% rely on it but you have a window from 35C to 45C, from cold start, 2C is a good approximate and you verify from scan tool, it comes out to be accurate.
Adjustment at 35C approximately and then at 45C even from oil pan temperature method is approximately correct to a 90% with temperature gun. that's far better results as compared to knowledge and what you get from local mechanic. Trust me, they have zero knowledge on subject matter. I have talked to many, all of them talked about adjusting it in start and that is done for them and told me that this is how they have been doing.