A 'fatty' would need to lose the weight first and gain some muscle. Wagner630 has the right idea.

body weight - body fat = leam body mass x 1.5 = grams protein needed

so calculation would look like this for me
im 201 pounds right now with 7% body fat. so this is how i would figure it out
201 x .07 = 14.07 pounds of fat
201 - 14.07 = 186.93 pounds of lean body mass
186.93 x 1.5 = 280.395 would be the amount of protein i need every day

I never believed in the whole 1-2g per lb of body weight nonsense.

a 400lb person should NOT be concerned about their protein intake.

I have seen excellent muscle gains from 250g protein. The secret is my carbs are at 600 and fats at almost 200.
More carbs means your body won't be breaking down as much protein.

The problem people think more protein = more gains. This isn't necessarily true. If you aren't getting sufficient carbs or fat in, most the protein you eat gets burned up for energy leaving you with little to no muscle gains.

CARBS = MASS GAIN. The added protein is good but you don't need 800g to gain lol

truth in the matter is that either approach would work. if you new your BF%, you could run the recommended intakes according to LBM rather than total weight. this would dramatically lower BF% over a short period. problem with this approach is that energy levels will be sacrificed.

the great thing about protein is that it is not very easily stored as fat. it must undergo several conversions and chemical reactions before it become a single fatty acid, which then must be bundled with other fatty acids to form a triglyceride, and stored in fat tissue. all of these processes burn energy.

so, the math remains the same regardless of LBM. for a higher BF%, carbs would be supplemented with additional protein. these supplemented proteins will be "deaminated" to yield glucose for energy purposes. once again, burning energy to make energy.

This has been debated for years. Its kinda like "supply & demand". Your body will absorb/ingest as much as you can eat. If it didn't and your body was at its dly max and then you eat a 16oz steak, you would crap whole pieces of meat. This never happens because you absorb/ingest everything you eat. Now if you are a lazy ass that sits on the couch playing Xbox all day your demand for protein is nil so your body will store it as fat. On the other hand if your hitting the gym 4-8 hours a day. Your demand for protein to build muscle is high so as fast as its absorbing its being put to use. This is also why there is no such thing as overtraining only under eating. Its not just your size but how hard you are training. How much protein can your body use? How dedicated are you?