Just eyeballing it, it seems you might put two caps in series and have greater total capacitance than with a single cap at 55V.
- Single cap at 25V (1.33 + 0.8) / 2 = 1.065 uF
- Single Cap at 55V (0.42 + 0.35) / 2 = 0.385 uF
- Two caps in series at 55V
A better approach would be to use a curve fitting routine to get a least squares curve fit, then calculate the capacitance vs voltage.
Or just put two caps in series and measure the capacitance.