custom pro platinum velocity

There is on "normal" velocity. The only way to tell the velocity of your marker is to chrono it.

You should chrono before every day of play, and every time you change tanks.

 You clearly didn't absorb any of what Snake just said. The only way to make sure a marker is safe is to crono it before use. There is no such thing as a number of turns that make the gun safe to use automatically. For all you know your mainspring is still to stiff and that ups the velocity far past where it should be. If you can't crono your marker before using it, then the only safe thing to do is not use it.

Go to your local PB place and ask if they can crono it. Get your tanks filled or something and they'd likely do it for free (using your own balls of course)