There are a few tools to convert Mb to bits, bytes to bits etc, but I like this one – http://www.matisse.net/bitcalc/

Directions are as follows:

direction = input or output
shaped = Speed you want to traffic shape in bits
normal burst = (shaped / 8) * 1.5 
extended burst = (normal burst * 2) 

For example, if you would like to ensure customer has 100Mbps speeds, you would do –

rate-limit input 104857600 19660800 39321600 conform-action transmit exceed-action drop
rate-limit output 104857600 19660800 39321600 conform-action transmit exceed-action drop