Tradator, I calculate my own hedge ratios, and agree that the SPAN calcs can get 'chippy' from time to time. Even if you use a moderately different hedge ratio, SPAN will still automatically calculate a margin credit for you so not to worry in terms of performance bond margin. My advice would be to monetize the last twenty days of trading ranges for each leg and use that - you are essentially accounting for volatility and tic sizing differentials in the same exercise.
EMG, FWIW, we have been killing the grain spreads. I have a client who hired me in February ( S&P Floor Trader) who is 7 for 7 this year in the grain spreads. We are pretty much out of July and are looking more at Sept - Nov, Sept - Dec, Sept - Mar12. The guy put his seat up for lease, set up a home office, and of all things bought the professional Bloomberg terminal. Says he wishes he had done it ten years ago.