Hi
I do not know X-Feeder at all, so I hope someone who does will provide expert help for you, but I think what you are asking is possible.
I have had a quick look at the X-Feeder manual, and although I cannot write the actual trigger file for you I think I can give you an example:
So here we will back Position 1 (according to Betfair's list)
IF the sum of amounts available for all three best lay prices on the selection is greater than the sum of amounts available for all three back prices
AND Position 1's volume% traded in the market is greater than 40%
Action: Back
Amount: back_size
Price: back_price
Conditions:
Selections Index in the game is equal to 1
AND Trigger Expression s_1_lay_amount+s_1_lay_amount2+s_1_lay_amount3 is greater than s_1_back_amount1+s_1_back_amount2+s_1_back_amount3
AND Trigger Expression s_1_volume is greater than 40
explanations:
lay_amount, lay_amount2 etc. are the volumes you have indicated in your screenshot
s_1_lay_amount denotes the best lay amount available for Position 1
s_1_lay_amount2 denotes the 2nd best lay amount available for Position 1
etc.
Hope this helps a bit.