Question: I need help forecasting sales revenues – any advice?

I’m trying to financially model and write a business case for an inbound/outbound contact centre. I can build up the estimated cost base for the agents, IT, building, hours of operation etc etc, but I have no idea what assumptions and ratios are acceptable to use for forecasting the sales revenues.  Is there a benchmark ratio of “sales per agent” or “total contact centre operating costs as a % of total sales” that I can use as a starting point?     The products for sale would be electrical goods (retail) with an average value of between 200 and 600 sterling per product sold and I figure the average handling time would be around 6 to 8 minutes.  Not sure what the call/sale conversion rate should be however.  Thanks.