IF it stays gas inside the tank rather than liquifying then this is easy: To put gas into a tank you need to pressurize it. The more you put in the harder it gets. "capacity" probably means how much you can safely put in before it bursts! Therefore at 80% it must be at a higher pressure and leak quicker.

It would leak more at 80% because of greater pressure

Same rate. You would just take longer to run out.

The pressure does not change, so it would leak at the same rate.

If the leak were the same identical size, the leakage rate would be identical. Pressure, inside a propane tank, is the same, no matter how many gallons it containes.

Get that leak fixed what ever the size. !!! The supplier is required to complete a leak test on the system. There should be zero leaks with the tank or fittings.

First I must cover my but! Any leak needs to be repaired, the chance of ignition is slim but if it does it could be catastrophic see links below. To quickly answer your question, it will leak at roughly the same rate no matter how full it is provided there is still some liquid in the tank. The rate at which the tank will leak is actually related to tank pressure rather than how full it is. Propane when pressurized will turn into a liquid (LP gas=Liquid Propane gas) and acts just like a refrigerant (actually a pure version of propane is R290 and is used in many small appliances overseas). Like a refrigerant the temperature of the tank will dictate the pressure of the tank. For instance as long as liquid is present in the tank at about 30 degrees F the tank will be about 52 psi, at about 100 degrees F the tank will be about 174 psi. Provided the tank temperature is constant the pressure in the tank will not change; therefore the leak rate will remain relatively constant regardless of how full it is. Once the liquid is gone this is thrown out the window and pressure will act more like a normal air compressor in that every time you use a bit the pressure will drop till there is no longer enough pressure to run the appliance you are using. As long as there is liquid in the tank the warmer the tanks surroundings the warmer the tank will be and the higher the pressure will be. The more propane that is currently being drawn/used the cooler the tank temperature will tend to be because as the liquid turns into vapor it absorbs heat from its surroundings. This is much more noticeable on a smaller tank like you would use on a gas grill and nearly unnoticeable on a larger tank you would have in the back yard. I do not like scare tactics but if the videos below save someone an injury it is worth it. Remember these tanks hold an enormous amount of energy. The average 500 gallon tank in the back yard when full holds over 36 million btus, that is enough energy to heat an average home in Wisconsin for over a month in the dead of winter! Overfilled 20# gas grill propane tank fire when pressure relief opens http://www.exponent.com/propane_tank_fire/ Controlled 500 gallon propane tank fire http://flashovertv.firerescue1.com/Clip.aspx?key=71CB954F9EF1B635 Uncontrolled large propane tank fire http://www.break.com/index/tacoma-propane-tank-explosion.html