Bottom bracket and chain guide... Please help.

[trxrincon]

Right folks, I will be running an Mrp chain device on my bike so I'm wondering what length the bottom bracket taper needs to be (un55) as I'll need enough room to mount the device boomerang in. I'm thinking 118 for the taper length but if anyone could help confirm this that would be great as not 100% sure it may need to be longer? Cheers Rob