Make a back or lay trigger and apply it for favourite selection only.
Use this conditions
r_2_back_price - r_1_back_price is greater than 1
and
r_1_back_price is less than 2
to identify a "clear favourite" as defined by your initial posting. You should also add a contidion for back book percentage, so that your trigger does not fire bets into a market which is illiqiud or openening, so add something like
and
b_book is less than 105
I hope this is useful for you.
Martin