Keep your multis short and sweet is my suggestion.
2,3 or 4 selections only.

If your selections,for example,are calculated as being 50% chances and bookies are giving 45%,then you have an overlay of 10% on each.
If you combine say 3 of your selections in a multi,your calculated price would be 12.5%
And the bookies would be offering 9.1%
So the overlay with the 3 runners in a multi is 27%
You have increased your overlay but certainly decreased the probability of a collect.
Maybe you didn't understand?.Would you like me to clear up any misunderstandings,or would you prefer if I just clear off!