I would create an SD timeline but use the Prores 422 codec. Then take your HD clips and drop them straight into the SD timeline. You can scale them to full frame or leave letterboxed. See what you think of the results.
I\’ve tried many of the compression formulas suggested here on the COW but like the results of what I\’ve suggested.
Jim McNally
The Commercial Factory
http://www.commercialfactory.com