I don’t think you can really set forecasts based on number of leads. But you can set goals, with some forecasts attached. These, In my option, are what they would be looking for. And, this is the first thing I ask a GM.
First thing is to look at is staffing - how many people actually sell cars, and what is your lead to salesperson ratio? This can be debated in great detail (and has A LOT of variables), but I really think that an internet sales person should not handle more than 75 leads a month - and by leads I mean UNIQUE CUSTOMERS, these include internet and PHONE if your department takes inbound phone calls.
So forecast 75 leads per sales person and then back out the numbers for solds:
75 leads = 30 appts = 22.5 appts shown = 11.25 cars sold
This is based on a 40% appt to lead, 75% appts shown, and 50% appts sold
Adjust based on your needs, lead quality (you should be tracking your leads in this same manner), and close rate. I feel these are pretty solid and obtainable numbers, however your mileage may vary. And, if you are not generating enough leads for your salespeople - then look at getting more leads.
If your numbers are good - this is also great logic to use when asking to hire another salesperson. Being able to explain and show what you can forecast for that person really helps justify the cost.