I was just using the formula you mentioned. So you would be using a quadratic interpolation, which I asume would be a good approximation. The formula would then be :Level = 0,000287*pressure²+0,613*pressure.
I had to adjust the numbers a little and tried this formula, which gave a curve that goes negative for the start of the graph and since I can't have less than 0ml in the reservoir I'm not sure where I failed.
The pressure with 500ml in the tank is 800mbar, with 900ml in the tank it's 1023mbar (the maximum the meter allows for).
500=a*800*800+b*800, 900=a*1023*1023+b*1023
X Y
--------------
0 0
68 -14
136 -18
205 -11
273 6
341 34
409 73
477 122
546 182
614 253
800 500
682 334
750 426
818 529
887 642
955 766
1023 900
I've topped the tank now to check the maximum pressure, but it will take some time before the value is steady.
And I forgot that it has to be corrected for the actually atmospheric pressure.