Making a company business plan is tricky. Nobody can predict the future. Everyone can argue about the validity of a point estimate of sales in year 5: $45m…
I tend to remove as many of these hard coded point estimates as possible from my spreadsheet, and instead build the whole thing up from drivers or factors that ‘you can touch’. Instead of a hard sales number it is # of potential clients x market share x price per client.
Here is my approach, starting top down, then building a bottom up model
Start with the hard coded number (top down)
Back out of the number the implied number of clients, price per client
Make an assumption what market share that number of clients implies
Now hard code the market size, market share, and price numbers and tweak those to calculate the top line number
Now that you have set the revenue forecast, apply a similar approach to cost. Given the amount of clients you need to get every year, how much would it cost to get them? Sales trips, sales people, conferences to visit etc. etc. How much would it cost to support each client? This is a much better cost planning tool that simply adding a number in your P&L.
The result of this exercise is that you get a dynamic company model which logically makes sense, the only wild card being that number of clients that you put in.