I'm not much of a trekkie, so this could be totally off-base
iirc, in the original series, the warp factors worked like so:
1=C, 2=2C, 3=4C, 4=16C, 5=256C etc... a square progression
and, in the movies and TNG, worked like so:
1=C, 2=2C, 3=8C, 4=64C, 5=4096C etc... a cube progression
like I say, I'm not a Trekkie, so I could be totally wrong on this
In the original series Warp was a square progression (btw, that means W1=1C, W2=4C, W3=9C, W4=16C, W5=25C)
In TNG, it's an exponential curve with Warp 10 equaling infinty.
IIRC, YMMV.