I think the calculations are a tad squiffy.
When you say "8 colours, at 1 bit per colour", isn't that not how binary bits work? 8 bits is 256 possible colours, while 8 colours is therefore 3 bits.
So, my maths (which admittedly may not be following the right route) would be:
(47*51678.72*3) to get bits. Then of course divide by 8 for bytes, and divide by 1024 for KB, which would be 889.4 KB
I hope my maths is on the right track there.