Why not just select 'Business or Higher' under the 'Class' option under 'Other Options'? (as per post 5 of the Ultimate Guide) in the main ITA tool.

Prices for I / J / D do not tend to change by day, only by season. Search for dates that roughly correspond to where you want to go and (if you do NOT tick 'Look for flights with seat availability') you'll get the I or D fare. You'll then know what city is best and can play with ba.com to find dates with I / D available.