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How to measure systematic risk of a stock?
Beta is the correct answer.  It is THE measure of the risk relationship of a stock with the broad market. R squared is incorrect unless you mean something very odd by "co-efficiency."  A stock that goes up each time the market goes down has very low co-efficiency (negative risk as you have defined it) but very high R squared.  A stock that goes the same direction as the market but twice as far (with a lot of noise) has a very low R squared but contains a lot of market risk.  A stock that always goes in the same direction as the market but only a 100th as far is very safe but has a very high R squared. You can calculate beta using "slope" in excel or doing a regression, but the easiest thing is just to look up the beta in yahoo finance or elsewhere.  You don't need to calculate it for yourself normally.