Help - P/CF to Valuation of a diversified company

hey guys - how do you value a section of a company based on a P/CF metric if it's a diversified company? i have the 2012 revenues split out by divisions, and i have nothing else split out by divisions - only EBITDA and such by the company as a whole. in the end, i want the NPV of division 2, but how do i get there? research doesn't break it out here.

would it make sense if i just calc the % of sales of the company, i.e. 10% from division 1, 50% for division 2 and 40% for division 3, and then just take the EV multiple that with the % of revenue it generates? i.e. EV = $100 million * 50%, hence, division 2 would be valued at $50 million? does that even make sense to kind of use a EV/sales metric?

i was given a hint to maybe use P/CF as a metric, but i don't know how to get there if CF is not broken down by division.