Thank you very much for your reply.
It looks like we took a similar approach. We could not change the fiscal year variant for the book depreciation area for various reasons, so I made a copy of Fiscal Year Variant 24 - Half Periods and added the period shift for our fiscal year which ends on 6/30 (OB29). As in your case, the fiscal year variant for book is year dependent, but the new one is not. I didn't think it mattered because the beginning and end dates line up. I then assigned the new fiscal year variant to each of the tax depreciation areas (MACRS, AMT, ACE and SMACRS) and regenerated the period controls (OA84). The company code itself is not set for half months as in your case.
Standard depreciation key MACRS MSTL should now calculate a half month's depreciation in the first month. Unfortunately this happens only if the capitalization date occurs in the second half of the month. A capitalization date in the first half of the month results in a full first month's depreciation, which is not correct. I have not changed the standard delivered period control method (010 - 03/03/03/06) for the depreciation key. Did you have the same problem?