Environmental limit is based on ISA+35/39.4/40 depending on the manufacturer, so for a sea level airport the standard ISA temperature is 15C, add 35 degrees to that and you get a limit of 50C, thats the maximum OAT that you can takeoff off with. A Boeing/Airbus would give you a limit of 54C.
Assumed temperatures are a way of fooling the engine into thinking thats its hotter, therefore you will get temperatures up to 80C but the above OAT limit would still apply.
ISA temperature decreases at a rate of 2C (1.98 for the purists) per 1000 feet, so for an 8000 ft airport, ISA would be 15- (8x2)= -1. Adding +35 would give you a maximum OAT of 34 degrees for operating from that airport.