This equation will also take into consideration a higher quota for senior sales people who have a higher base salary. While just dividing base salary seems easy, it does not take into considerations many factors that you need to consider. The first factor is your product set. The opposite is true also. The other consideration is what level of sales are actually possible both given your geographic and market area, and your system limits.
Quotas are there to be both a goal and a measuring tool.
If a sales person is constantly missing their quota, and the rest of the sales force is beating it, then it is probably time for that sales person to shape up or ship out. If no one is beating the quota, then maybe it is a little high. You need to experiment with quotas to see what is right for you. You should also have an incentive for sales people to beat their quota. Sales people are competitive people, and contests work.