RSI can be a very useful oscillator. And it's designed to show a stock's relative strength in relation to itself. In other words, how is the stock doing now compared to how it's usually doing. These days calling it a Relative Strength Indicator is misleading because most people interpret that in relation to an index.
Your software should make it very easy. On your level II box it should have the following. ChgOP and Chg. or something like that. That way when you are flipping through stocks you immediately know hey COF (for example) is down 4.69 percent from the day and 3.2 percent from the open. That way you can pull up each stock real quick and know which is the weakest. It also allows you to play one stock of the other...and pair up stocks. If your software doesn't do that then complain about it. Stuff like that should be standard. Nobody should be guessing....hey msft is down 3 i think that is like 5%. You should know to the %. Time is money. I did enough math in college.
Simplest way I know would be to calculate (price-price(n)/price for all the issues and the e.g. $ndx. That shows you how much prices have advanced over 'n' bars on a relative basis. Then just pick the highest values, assuming that's what you want to do. This has nothing to do with RSI, which measure price movement relative to itself.