First step is to locate your thumbs control valve in your main valve stack and see if it has port relief valves. Usually Allen head set screw and jam nut , that’s where you’ll set the relief pressure for your thumb base end .
You can plug a 5000psi pressure guage into your quick connect that supplies thumb cylinder base port. Read pressure as you operate the close thumb button.
Counter clockwise rotation of set screw reduces pressure, doesn’t take much maybe start with 1/8 turn and check.
Do you have a recommended relief pressure? If not main thing is to just set it less than your thumb cylinder pressure rating.
Here’s a picture of the port relief valve for my thumb cylinder on a Volvo.