The wattage tells you the rate of heat input. But it is difficult to know the rate of heat output. The temperature will depend on the balance.
The rate of heat loss will depend on the size, shape and materials of the iron, as well as the environment in which you are working.
You could experiment to determine the rate of heat loss, but how would that help, if you can't vary the input wattage? If the wattage of your iron is too high and you buy a lower wattage iron,, but it's rate of cooling could be different
If you can vary the input power, then simply adjust it to give the right temperature, or at least, a temperature which makes you happy.
The ideal iron, IMO, would have a very high wattage and be controlled by temperature feedback. But if you had an iron with this, you probably wouldn't be asking the question. I don't have such an iron, so I provide the feedback manually.
IMO there is not a single temperature that is right for everybody, for all irons, for all circumstances. The temperature you need is above the solder MP, such that the tip does not fall below the MP during the soldering operation. The power you need is that which will return the tip to this temperature between soldering operations. If the desired recovery time is short, you need higher power. For casual use, you can tolerate a longer recovery time and so lower power. If you use a high power then reduce your rate of soldering, the iron temperature will increase.
When I set up, I set a low power until I am ready to get soldering. Then put it up to high until it is hot enough - melting solder v.quickly while I'm washing the tip in solder - and then throttle back to a working setting, based on experience. That level depends on the materials you are soldering. 0.5mm wire to a 2mm pad needs a lower temperature (or smaller iron) than 16 gauge wire to 5mm tag. Three transistor wires are going to cool the iron less than a 28 pin dip holder. Since I normally solder only a few joints at a time, I lower the power between spells of activity, then raise it again when I'm ready to recommence.
The only way I can judge the correct temperature is by the immediacy of the solder melting, the flow of solder over the joint and the appearance of the finished joint. I have no idea what the number of degrees is.
As for the tip oxidising, my suspicion is that you may not be tinning the iron well. If the tip is well coated in solder, that should protect the iron, even if overheated for a long time. If you initially do tin the iron well, try ensuring that you give it a wipe and recoat it well before leaving it idle.