Is there any difference between safety valve relief pressure and cracking pressure and how to adjust these pressure
Yes there is a difference. Relief valve pressure is a full flow pressure. Cracking pressure is a lower pressure where the relief begins to open. IMO, cracking pressure can only be set correctly on a test bench with a known flow to achieve the pressure reading when the relief begins to open. You can get it sorta close on a machine, but it takes experience, patience, and time to get it sorta close.
So why would this be an issue? The only time I've seen a problem is when the safety valve is bad to start with and you can't get it to set high at full relief.