On average (not for sure, just an average) lamb dress out at 51% so that X 150 = 76.5 lbs.
This figure will then need to be multiplied by 75% to get take home meat = 57.375 lbs. take home.

See this publication for more of a discussion on all the factors addressing meat cut out and percentages:
http://www.caes.uga.edu/topics/sustainag/documents/didthelockerstealmymeat.pdf