There is a video of a michelin star chef talking about cooking fish. For him, a good seared fish is the sign of a food kitchen. It cooks faster, and overshooting the target internal temp ruins it quickly. The margin of error is a lot smaller.
So, in general my first tip is, heat travels at a roughly constant speed through the food. If you are searing on one side only, it will cook through 1inch of meat/fish at the same rate, regardless if you are using medium or high heat. Just the skin will be more or less burned.
This also means all time measurements are depending on the thickness of your fish. In general I advice to cook salmon on somewhere around "medium" heat. This also means, your perfectly cooked fish is cooked at different temperatures depending on its size, if you want to nail the internal temps.
And nailing those internal temps is what the chefs train for. Wild salmon should be taken above 145F internal to kill parasites. But if you have salmon that can be eaten raw, you can take it lower. Around 43c/109F to 45c/113F max The internals become translucent. Its my favorite cooked salmon if fresh. If you cook to FDA, I'd see that you are as low as possible, as evenly as possible. So develop a sear, and move it to a warm, i direct side for a few minutes. Keep carry-over heat in mind. If you cook your salmon to 145 on the grill, it will be easily at 155 until its sitting on a plate.
And yeah... accept failure. I grilled a young whole salmon recently. And it was a disaster. The parts that were cooked were overcooked. Parts that were under-cooked were raw. Too much heat and too little time. Good fish is an art.
