1. Average invoice? How would you think that shoudl be calculated? I wouldn't think you would want to evenly divide the sales between each account based on a total amount to reach but instead that it should somehow be proportional. What about new customers or lost customers? Do you create a report like this today and if so - how do you calculate the per rep values? Are you going down to the account level as it shows here or do you roll up to the sales rep?
2. is this just Target minus current sales.
This thread talks about forecasting but forecasting isn't really a target - it's a look back over time based on previous activity to deterimine what is likely to occur. I'd be surprised if any sales org used forecasting like that as a motivation for what the sales team should hit. Don't you want sales to grow faster, higher than previous? Anyway - a bit more information about the use case and some more sample data (ideally in a workbook) and I think we could probably be helpful. The forecasting tool doesn't sound like the right thing here because you won't be able to use the forecast values in calculations. Instead, creating a target based on historical volume/amounts/sales with some percent growth computed for the next 6 months is probably a better way to go.