How do you calculate percent error and the Theoretical Percentage of something?

Need more than just a formula...I need an explanation too. In detail...we have to calculate the theoretical percentage of water in BaCl2 x 2H20...and then compare the percent error to our other calculations.How do you calculate percent error and the Theoretical Percentage of something?
BaCl2.2H2O molar mass =

(137.33 + 2 x 35.453) + 2 x (2 x 1.0079 + 15.9994) = 208.236 + 36.030 = 244.266

244.266 grams (1 mole) of BaCl2.2H2O contains 36.030 g H2O (2 moles of H2O).

36.030 / 244.266 = 0.1475 (14.75%)



Assume you found 15.21% H2O; your percent error is (15.21 - 14.75)/14.75 = 0.031 (3.1%)



Assume you found 13.96% H2O; your percent error is (14.75 - 13.96) / 14.75 = 0.054 (5.4%)