You taKe the wattage of the CPU and max wattage of the gpu. Add them together that the wattage that has to be on the 12v leg. To be safe make sure there extra 50 to 100 w on the 12v leg for cap aging. Then look to forums that have power supply load tests. Min power supply now in a gaming rig is a bronze level power supply. Top level would be a gold or platinum level unit. There are vendors that are high rated like seasonic units. Under that vendor will be two or three Oem that sell the seasonic units rebranded under there own name. Bad or unknown units like deabloteck you can google and see them fail and fail with good loud cap pops. Most of these fail because there 300w units relabeled as 500-600w units or they can't output there rated voltage without blowing up.
Best failure was a Canadian pc tester that was testing a piece of junk.
When the ps failed it took out the load tester and shorted out the building.
It is not wrong to overprovision a psu a bit, and it usually costs little more.
A psu will only consume the wattage it needs regardless of it's maximum capability.
A psu will operate most efficiently in the middle third of it's range and be quieter. A 20% extra is not a bad idea.