I have come up with this idea.
You have the fields in front of you.
Starting with the first leg,you briefly sum up the winning chances of the first three TAB numbers,and eliminate the worst of the three.
Then you do same with Numbers 4,5 and 6.
And 7,8 and 9.
If the first leg,for example,has 10 runners,you then sum up the winning chances of number 10 in the first leg and number 1 and 2 in the 2nd leg.Eliminate the selection with the least chance of winning.
The comes 3,4 and 5 in the 2nd leg.
Continue like this until the last leg and sort of start again like comparing 3 horses.
Always if you have only 2 or less,then add a horse from the next leg.
Keep crossing out horses 1 by 1 until the combined selections cost whatever you are prepared to outlay.
If you have trouble eliminating a selection out of the 3,then pass and move on to the next group of 3.
Not very Scientific,but can be fun and rewarding.