As you've already taken all of the "analogRead vs. distance" readings and plugged them into your spreadsheet to graph them, just use your spreadsheet program's method of creating a line of best fit to those values.
Excel's function is "Add Trendline" and OpenOffice Calc calls this "Line of Best Fit" and either option will be found within the Chart menu. When either spreadsheet calculates that line it will provide you with the formula you need to plug into your Arduino sketch.