Calculating Home Office depreciation expense. Do I add in all the home improvements since I purchased the home in 1988 to the cost of the home, less the cost of the land

Yes, you will add all capital improvement costs to the original cost of your home to determine the depreciable basis for your home office deduction. As you mentioned, you will need to remove the cost of the land (since land is not depreciable).

Please click on this link IRS - Home Office Deduction for more information.