Are you using the prefork MPM? Probably, so I will go with that.
StartServers are kind of too far away from each other. The other thing is, see what modules you have enabled and let only the ones you need, so you will have smaller apache2 processes: smaller processes mean more processes in less memory.
The other thing I can suggest you is to monitor the number of apache processes you usually have and tune from here. If you always run on a low 30 and high 80, you can keep the spare servers higher.
Also, take a look on the keep alive configuration, you may want to get your
MaxKeepAliveRequests higher and maybe get your
KeepAliveTimeout lower. If you start to get too many zombies you can maybe turn off keep alive entirely or set the number of requests to a slow number (so the apache children will be recycled faster) but there's a performance hit.
Anyway, there's no way to exactly tune it without monitoring and seeing if there's any bottleneck to be solved on apache or the OS. Take a look at
vmstat and check the number of processes, what they are doing and so on so you can identify choke points.