...subreddit that explains it all in a lot of detail that I actually pulled the data I used.
Realistically, like you said, there are different types of speed, so this is more of just a general guideline, it was more trying to provide ballpark ranges for those not familiar with track times and how they would relate to 40 times.
Also, Gatlin Bair actually has a fairly "slow" 40 time when compared to his 100M... according to the calculator, he should have a mid 4.2 40, but he was clocked at 4.44 earlier this year officially.
Here is the conversion calculator: