Booney, you're on the right track. 2.5X mangification is about right for what you want to do. However, I don't know of any mfg's. of 2.5X scopes. You pretty much have to choose from 3-9X, 2X or 4X. That's magnification power.

Then you have to consider the lense "objective". You said 2.5 X 20. A 20mm lense is kinda puny. You would probobly be much happier with a 30, 40 or even 50mm lense as the larger the lense the more light the scope can pull in during low light.