Convert a Hex value to measurement value

Your question is application specific. There is no general answer for that. A proper answer depends completely on your following the forum guidelines in how to post a question properly

Because:

No it is another value
DEC 75 is -30

Why? Do you think this a common relationship? It isn't. So please be considerate of our time, and fully explain and reveal where these numbers are coming from and what they mean.