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A simple formula would do, how about something like..
Charge time (Ct) = [Time] per [unit of distance] multiplied by (1 + [ship's SP as a percentage])
or something geometric like the above formula but... Ct = ...([ship's SP as a percentage]^x))
x being equal to any number greater than 1.00 and less than or equal to 2.00 that you feel yields an appropriate geometric progression as size increases. Just keep the exponent the same for every ship.