Hi ianrich77,
to IIS!
There are two main things that govern the image quality seen. The first is the optics, and the second is the atmosphere.
First optics:
The rule of thumb for maximum magnification obtainable by a given diameter of apeture is:
50X per inch of apeture.
So for your 5" scope, this would be 250X. This is regardless of focal length! It is a factor completely governed by the size of your apeture. Your may push it to 60X per inch but then the quality of your optics needs to be really good. Here we deal with the real world.
Second atmosphere:
The majority of the time, the highest useable power will be between 100X and 150X. The reasons is turbuance in the atmosphere becomes much more readily visible the higher in power we go. The ocassions when the atmosphere allows for a good stable image at higher than this range diminishes the higher we go. Even so, at sea level, we are usually limited to 300X.
Don't feel short changed. In the last year, I've only had 2 occasions when I was able to use 360X on my 17.5" scope to view Jupiter for a good image, just. Theoretically I should be able to go to 875X. I've got no chance at sea level.
Also, the hotter the day, the worse the turbulance. Most stable viewing occurs in the early, early morning.