The sensor I use compensates for local pressure because it has an inlet that DC biases the sensors output based on the pressure it reads, so in reality its 2 pressure sensors. 1 to measure ambient pressure and modify the DC level of the sensors output, and then the actual pressure sensor used to measure the pressure of the "depth". Its the default sensor the brew troller guys tell you to use, works great.
As far as the 1/10 of a gallon that number includes variances in gravity due to change in efficiency. If you know exactly what your gravity is then its accurate to within .002 gallons.
And yes, its a 12 gallon preboil for a 10 gallon batch.
As far as flow rate, I found a set of turbine based flow meters for only 40 bucks a pop, they gave a pulsed output that you could count over time to get a flow rate. The issue was that if you get a larger turbine (larger diameter) the price goes up fast, and they become harder to count (lower rpm for same flow rate), but if they are smaller diameter (these were 3/8 inch ID) I've had issues with them clogging due to all the stuff that is in wort. Thus I just went with the change in volume over change in time. I'd love to have a capacitance drive or ultrasonic type flow meter but they are just so damned expensive.
So the bubbler that the brew troller guys (and I) use compensates for local pressure, and temperature, then you have to give it your estimated pre-boil gravity to have it compensate for gravity ( I wrote that code for them, I believe they put it into the open source code base). I went through a crap ton of math just to show that you can just take the ADC value from the pressure sensor and just divide it by the gravity and it compensates for the change in pressure due to change in density.... all that work just to prove the fix was so simple.
I'll also warn you that with the bubbler the voltage output from the sensors is very noisy, I had to put a low pass filter in between the sensor and the ADC as well as use a 32 sample average with a 20ms sample time to get it to work so well.