I am having problem accepting the Harville method for calculating Exactas.

I have 2 animals from the Field I wish to calculate the Chances of the Exacta occuring

#1 is $2.50
#2 is $11

The Exactas work out as 1 - 2 is $16.80
and for 2 - 1 is $25.20

It seems to me that the Chance of the Rougher animal beating the more Favoured animal is too close to the Chances of the Favoured animal beating the Roughie.

If I could calculate each animals Chances of winning the race, and also calculate each animals chances of running second, is there a Formula I could use that would give me a more accurate assessment of the Exacta occuring.