If you're going to go the cake route, i think data suggests to remove *some* of the cake prior to pitching. The whole cake would be too much. Though, in the past i've used full cakes which have turned out fine. If you DO wash, you'd be best served to utilize a starter and step up a few times for the cell count to be plenty for a BW.
Regarding the equivalents (1 jar = 1 smack pack, etc.), i can't comment on that, as i just deal in increments of starters in quarts.