HI Kellmark good luck with what you are doing.
i find that if you do a 3x5 banker system with your best value runners you will come out on top at the end of the day. If the favs dont win.

i put them into single bets on paper and times the 2 win prices to see what it should pay, but before i do that i drop the win price by 1 point to allow for a downward movement in the win div.
so horse 1= $4.50 it is for me $3.50****horse 2 $6.00 for win now $5.00 times the two and it should pay $ 17.50 for the exacta. of course it depends on the exacta pool and number of tickets but it gives you a rough idea.

the best way is to do them with single tickets [ but it is a lot of tickets to write out if you have 5 or 6 numbers ]. because you can then stake them to there prices more on the low and less on the high.
i dont know how to write excel but if you do you could get it so you just put in your numbers at the top and it sorts them into single bets so then you can put in your prices next to each horse and let excel times each combo for you.
make life easy.

thats enough of my rambling on
cheers Rick