When you're determining the doneness by time, you're doing it wrong :)
There are too many differences between individual fish to go just by weight, thickness, oven temperature, and time. The best way is to measure the internal temperature (stick a thermometer into the thickest part of the fillet); I like about 120 F (50 C) for salmon (carryover cooking raises the temperature a little higher after taking it out of the pan), but then it has to be good quality fish. You need to check early and often, because it can go from undercooked to overcooked fairly quickly. Now, like zanlok says, my experience is mostly with pan frying, and I'd expect using the oven would be a bit slower than using a pan because of slower heat transfer; so you should be a bit safer.
If you don't have a meat thermometer, an easy way to test fish for doneness is to look for translucency. Salmon can still be just a little translucent in the middle. Also, properly done fish flakes along the natural "seams". Another nice trick (which doesn't work for boneless fillets - sorry) is to find a bone in the thickest part of the fish: if it comes out fairly easily, then the connective tissues have mostly dissolved and it's done.
That leaves the temperature for your oven. I would go with very high - that way the fish gets the least possible time to dry out (which is going to be a big risk with this cooking method, I would think - unless you wrap your salmon in bacon or something like that). At a guess, I'd try 450 F (230 C).
I would cook the Brie at that same temperature for 30-35 minutes and it should be okay.
Alternatively, you could cook the roast fully, remove it from the oven, tent it with foil and then cook the Brie. The meat can rest while the Brie is cooking and being eaten. Prior to slicing the roast, heat up the pan drippings to a sizzling temp and baste to ensure a crispy crust. But if the appetizer course happens to last for 20-30 minutes, then the roast will have been resting for 40-50 minutes and could lose too much internal heat.
Best Answer
Most ovens are not all that precise, really, and most recipes allow for that (your 5 minutes of variance on each item.) The top of the oven tends to be hotter than the bottom, so this works that by putting the cooler item lower.
For more drastically different items, you cook one, wrap it up, set it aside to hold, cook the other. You might undercook the first a touch before holding it and put in back in the oven briefly before serving, you might not, depending what it is and how much "hot out of the oven" is an important part of serving it well.