Ok, the formula for FAVORITES is below. Since a dog will never be -1.5, you don't need that formula.
The first thing you need to do is determine how much you want to wager. Generally the guys on here post in their number of units played recommendation or Rolltide has his X/10 with the X being the unit recommendation. Thus, if you play $50 units and there is a 3 unit play, you want to risk $150 total dollars.
=(100/((100/X)+1))*Y
X is the line for the money line (say Chelsea is -300, Sheffield U is +500 and Draw is +220), you would insert 300 (not -300) for the X
Y is the number of units you want to play BASED ON $100 UNITS. Thus, if you play $20 units and this is a 3 unit play, you would insert .6. If you play $150 units and this is a 3 unit play, you would insert 4.5
Now, if you are playing Chelsea and they are -300, you play $150 units and this is a 3 unit play the formula would be
=(100/((100/300)+1))*4.5
That will give you the amount of money you will win on the Chelsea money line, which is risking $337.50 to win $112.50. You then take all of your winnings ($112.50) and bet Chelsea -1.5.
Let's say Chelsea -1.5 is -$125, you would then risk $112.50 on Chelsea -1.5 to win $90.
Your total risk is $450 ($337.50 bet on ML and $112.5 bet on -1.5). Your results are:
Chelsea draws or loses = -$450, same as you would have risked in a -1.5 bet
Chelsea wins by 1 = +112.50 on the ML bet and -$112.50 on the -1.5 bet for a push
Chelsea wins by more than 1 = $112.50 on the ML bet and +$90 on the -1.5 bet for a total +$202.50
Thus, you risk $450 to win $202.50 instead of $360 but you don't lose on a 1 goal win.
Make sense? if you are going to do this, I suggest using excel, I can send you the formulas so that you only have to put in the number of units and lines to determine how much you risk on each.