On the outside of the boxes of r507 I've seen it has arrow pointing to the bottom of the bottle saying this side up for use. To me that means liquid charge and that's what I've always done. I would start with 50psi static charge. Start the unit and then slowly add in very small amount. Maybe a 2 second blast. Wait 10 - 15 min to give unit time to equalize out the charge. I watch for frost returning on the suction line keeping it back a couple of inches from compressor. After I get it in the ballpark I add the load I want and tweak from there.