The key is mostly to cook over coals rather than open flame if you want decent control. This is a principle you find all over slow smoking/BBQ. You start the fire with plenty of wood and let it burn down to a pile of red coals, which you then can cook over with nice control.
Personally, when I want to cook over an open fire, I treat the fire pit as 2 zones: fire and coals. I use the "fire" side for anything where I want an open flame, like roasting marshmallows or hot dogs. As a chunk of wood on that side of the pit burns down to coals, I move it over to the other side, for more controlled cooking.
Another way to handle the variability in campfires is to cook with things with a lot of thermal mass to even the heat out. In practical terms, that boils down to cast iron and clay/ceramics. Whether it's a dutch oven or a little "cave" of fire bricks, these materials spread the heat out so nicely you can often just bury them in the coals and "bake" or "roast" a lot of options that you might not have considered as "camping foods".
The same is true of other cast iron pans and much of the stoneware from places like Pampered Chef, meaning you can pan fry, saute, and do many of the other more typical "kitchen" tasks over a campfire.
One word of warning regarding this principle though is that it might seem like collecting rocks from a nearby stream to use as the thermal mass is a good idea. It is not. Rocks that have spent long periods under water can crack and/or explode when suddenly heated up.
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).
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That white stuff is albumen, the same protein that makes the white of chicken eggs. The albumen is part of the salmon's blood, which means that your fillet was fresh. A trick for dealing with it was developed by Bruno Goussault while collaborating with chef Michel Richard from Citronelle in DC: soak the salmon pieces in an ice cold brine. The article in the New York Times that told the story recommends soaking the salmon fillets for ten minutes in a 10% brine (by weight). To make the brine, dissolve three tablespoons of Kosher salt for every cup of ice cold water and make as many cups as you need.