Fumble your calculations are correct. In my opinion, at least.
However, if you reduced to 8 psi pressure you would have to repeat the test to find the gallons per thousand sqft. The pump output curves are not accurate enough for this type of thing.
So you are spraying about 17.4 thousand sqft per tank(if you got 3.45 gals per1000 sqft).
Using a larger area and a whole tank will help to average out any speed changes and give you a better number. Of course you need to measure the sqft covered.