I'm glad you got the idle to 800-850rpms.
But why did the engine temp almost get to H so fast just by adjusting carb idle ?
Did you fill your radiator with enough water ?! The level should be about 1" shy of the filler neck. Yes, Water Wetter is a good thing to add, can't hurt. Just don't add too much anti-freeze as Water Wetter works best with straight water however, you do need some anti-freeze for anti-corrosion protection. Less than 50/50 mixture is okay.
No, you don't have to leave the radiator cap off to vent.
Best to leave it on because as radiator/coolant gets hotter, the pressure cap actually helps suppress overheating.