40 Gbps would actually be exactly 5 Gigabytes per second (divided by 8).
At the lower speeds, things like 8b/10b encoding and Reed-Solomon ECC added enough overhead that dividing by 10 was more accurate than dividing by 8.
on 10gige pipes, at max Ethernet mtu (1500) bytes etc, there is approx. 94% of available bandwidth for user data (accounting for things like inter-frame-gap, crc checksums etc). with jumbo-frames that number goes to 99%.
That would mean that I would need to divide by about roughly nine (8.8 or so).
Sorry, I can't do divide-by-nine quickly in my head. I can do divide by 10 though, and my error is roughly 10%.