Mike > Estimate depends on ISO and final file sizes depends ISOs and subject/aperture too (f1.2 creamy = smaller files, landscape at f5.6 with trees = big ones). Using low ISOs and bright aperture all the time, my raw are ~20% smaller than canon estimate. So I know the camera estimate has a good safety margin for my use. Your mileage may vary.
My advice is to put two big cards in the camera and stop worrying about the size. I use 2x 64Go Transcend 400x UDMA 7 with 'write to both' (no brand is fully safe) and their cost is really low (I paid 15k JPY for both, that is about ~200 USD I think). At this point storage should not be an issue.
Transcend 400x are a sweet choice unless you do need the 1000x performance. See here :
Bingo...I'm estimating that the camera is underestimating by about 20% so this makes perfect sense...I currently have two sandisk extreme 64gb's in it...should be good enough for now...if they do feel slow I'll give the transend a try, eh.