go to
http://www.quilterscache.com
find the alphabetized list and click on the alamo. (under 'a')
the block is different, but it gives exact direction for this problem. if you follow their proportions, you will end up with the correct sizing and have your seam allowance. their solution to cutting is a great method that many of us have used with a lot of success. test it out.
editing here: remember - proportions only...not necessarily the same sizes.