Core temp below 75C is normal, Mem temp below 100C is normal. Ideally you should keep them max at 65C core and below 82C for mem. This is largely upto your personal preference, as long as its within the cards limits. Keeping cards low temp for longevity and stability is a part of mining. Look out if there are stability problems and if T-Rex shows your temperature values in yellow or red color, that means you should really decrease your temps somehow. Power limit setting will help you the most but you need to find that sweet equilibrium spot which won’t kill your hashrate much but also won’t use high power and therefore too much heat. This is represented in T-Rex as “kh/W”, higher the better. Using a locked core clock (at least with ethash) will make it more efficient. You could also start by manually setting higher fan speeds even 100% if need be, moving hot air away from cards & exhausting it from the room as quickly as possible, will help.
Up to 300w is acceptable with these cards but miners mostly keep them between 280-290w. If you’re pumping this much of current into your cards, you need to make sure of feeding it directly from power supply & not using sata cables and use high quality risers (at least 009s)
Every card has a overclock range of its own due to silicon lottery or in some cards due to different memory brands. So you need to make a simple search for 3080ti overclock settings. I suggest you use reddit for such inquiries. Or just google search “3080ti mining overclock reddit”
You also need to fine-tune your cards yourself after establishing a starting point.