Best Practice for managing your engineering headcount needs

Do you currently measure how much capacity your existing engineer's can handle? How many clients/work load? If you have this type of quantitative data it should help you gauge the necessity to add someone. I have a similar issue in admin. As we continue to add stores and everyone is working at capacity I hate adding overhead unless it's a necessity and I don't like relying on everyone to "tell" me they can't handle more. My philosophy is "show me" so each one of my team has a Task List with Times where they timed each job duty the best they could when they were hired on and once a year I ask them to update it because things change (duties streamline, processes change, etc). The great thing is when I feel the pain of overload in the office an I know we are about to grow I can make an objective decision about what position I need and who needs to offload some duties.

Now, I don't know if it's that simple for engineers to do as I am sure it's so random based on client/project/scope; but if you have some way of measuring output / productivity it should give you an idea of your needs. Hope this helps a little. :)