The options I see are:
1. Assume some volume of sales per sales person and then calculate total revenue based on sales heads.
2. Assume some revenue target and assign a quota to each salesperson and calcualte the number of sales people.
3. Assume some cost to acquire each customer, set a marketing space and calculate the number of new customers based on marketing spend/cost per acquiisiton.
Doing forecasting need to determine:
1) the sources of revenue and the revenue drivers
2) the classification of costs and the costs drivers
3) the nature of costs and their patterns
Then you develop profit and loss + balance sheet + cash flows forecast.
With the above basis, you can perform the cost of customer acquisition per type of revenue per year; the cost of customer acquisition & maintenance / total lifetime revenue for ROI or NPV; break-even, the margin of safety, contribution per sales type; ROCE; market sensitive test; etc.
If you adopt the above, you will somewhat be getting things in the proper place.