dr.ron,
it does seem obvious but you'll be disappointed with using last 12 months and/or recent x no. starts.
For about a fair while I was calculating an API over last 10 starts, and overall didn't find an 'edge' over career API.
I then used distance as a factor. So if a horse had won a 2400m race then was resuming over 1200m I didn't include it in the calculated API. The odd race seemed promising but overall again vs career API was disappointing. Even allowing for some 2yo days and high prized restriced races you'll find time spent on API other than career API won't be worth the effort. Of course please find out yourself, and give Chrome Prince's theory a test run. Maybe a Baghwan or Becareful might have some stats to help out.