Understanding Mcintosh Meters?

This is probably a stupid question, or atleast one I should be able to answer myself, being that I did graduate with a degree in computer engineering, but I recently got a mcintosh ma6450, and was wondering how I make sense of the meters. I understand what they measure, and what it means, but how do i know when I am close to clipping.

Let me explain further. the amp is rated 100wpc, in to 4 ohms and the meters read from .01 to 100 watts with the final mark being 200 watts. But what if my speakers are 8ohms? I know that means my amp produces about 50 watts in to 8ohms, but does that mean that I can only take the meters about 1/2 way before I will clip, or do I go by the decibels, meaning 0db is really the max before clipping, only instead of producing 100wpc (0db corresponds to 100w watts on the meters), the amp is only producing ~50wpc since the speakers are 8ohms.

Hope I explained my question well enough. Thanks for your help.