Asset value for Calculation of Tax Depreciation

Question

We have been told that many of the assets created as of 1/1/2008 have a different (lower) tax value than their beginning book value.  Book value is correct for standard book depreciation.     Is there a process to update the tax value of assets created in SAP similar to the transaction AS92 used to update values for Converted Assets?
It was suggested that we use the method of Unplanned Depreciation tcode ABAA with TTY 642/652 to change the tax value of the asset.  When I tested this I found the various tax depreciation methods were calculated based on the beginning book value for year 1 and not using taking the special depreciation into consideration until subsequent years.   This is using Depreciation Area 10, MACRS, Dep. Key M200.
Is there a better method of altering asset book value for tax purposes or a better way to alter the tax depreciation calculation so it takes Unplanned Depreciation into consideration during year 1?
Current simplified tax depreciation example:
Beg. BV $1,000
unplanned dep posted $100
tax depreciation for year 1 $100
tax depreciation for years 2-10 $88.88
The preferred depreciation is $90 years 1 - 10.
Thanks for your assistance,
Jeff

Answer 1

I think I had the cart before the horse on this problem.  The correct way to look at this is what is the best way to get the correct "Net Book Value" and not the correct tax depreciation posting as it never actually gets booked.  
By knowing the incorrect Beginning BV, the rate depreciation is applied in year 1 and finally the desired Net BV value at the end of year 1 for tax purposes I am able to calculate the correct Unplanned Depreciation for tax to make everything work out correctly.
correct beginning tax BV $1,200
Beginning Asset Book Value $1,800
depreciation $360 (20% of BV)
Unplanned depreciation  $?
Net BV for tax at end of Yr1  $960 ;  or  1,200 - (1,200*.2)
With the know values above that makes my unplanned depreciation for Yr 1 to be $480.  Year 1 and all following years depreciate correctly after the Unplanned Depreciation is posted.
I am marking this as resolved unless someone sees an error in my thinking.
Thanks,
Jeff