For getting floor space factors (numbers of users per meter) you can use the guidance of part B as a rough guide. However a better option to help you visualise the space is to refer to guidance in the metric handbook and to log on to a few seating manufactures websites to look at their standard drawings.

Regarding water proofing I would also go for a drained cavity system but If you look at tanking manufacturers web site there are lots of details and information for you to use (I would suggest a few but don't want to recommend one over another - try to use the RIBA product selector or just Google!)

Good luck and if you get really stuck you can always try to measure up an existing lecture theatre.