On average the goal should be 150% of your average attendance. For example if you average 150 per week than you should be seeing 225 visitors per year. Break that down to about 19/month or 5 per week. If you have 5 visitors a week than 5/150= 3%
The rest of the stats state 25-50% of those visitors actually give you info. 20% of those who give you info actually make it through Growth Track or whatever process you have in place to take visitors to volunteers.
Hope that helps.