Calculate the slope in absolute units then divide by the ATR (average true range) or a similar measure of volatility.
(Linear Regression Slope) gives you points per day. 100*((Linear Regression Slope) / (today's close)) gives you percent per day. Probably this is what you want if you are comparing two stocks. (Linear Regression Slope) / (5 day EMA of True Range) gives you ATR's per day. Probably this is what you want if you are comparing two commodities, especially when using backadjusted continuous contracts.
This is fairly close to what your looking for. http://gummy-stuff.org/raff-regression.htm or look around is site, tons of stuff. Gummy Rules! http://gummy-stuff.org/ all spreadsheets can be downloaded for free.
http://phoenix.phys.clemson.edu/tutorials/excel/regression.html That may help... to do it in percentage, just calculate the percentage change in the last column and use that rather than the $ change. - The New Guy
Use the log of the price series rather than the price series itself. That way the slope of the linear regression equals (Δprice/price)/Δtime rather than Δprice/Δtime.