Hi,
Information on how to calculate CCA is located in guide
http://www.cra-arc.gc.ca/E/pub/tg/t4036/t4036-09e.pdf
at pages 17 Land and 21 Area F.
Separating the purchase price into a land and building component is always difficult. If the vendor has not done this with you when you bought it, you may want to look at 1) the property tax assessment (do they break down the respective values?) 2) ask a realtor familiar with your area on the value of lots like yours 3) do a best estimate yourself and allocate it in that manner.
Since you have identified that previous tax returns are incorrect, you could
1) adjust your previous years returns to exclude the excess CCA that was deducted (the best scenario) Using T1-Adj
http://www.cra-arc.gc.ca/E/pbg/tf/t1-adj/README.html
2) leave the incorrect calculation as is knowing that if you are audited they will adjust it BUT keeping in mind that CCA taken in these previous years reduces the ending value or UCC (Undepreciated capital cost) of the buildings alone. If ever you sell this building at a higher price that you bought it CCA deducted will be recaptured.
Action 1) is the one that always ought to be taken but action 2) is observed in actual practice given the difficulties breaking down the land/building values.
Hope this helps.