What I usually do,Floydyboy,is assess what price those $1.50 shots would be if I was a bookmaker.
So maybe I think they should be $1.20
Then I multiply the $1.20 or $1.15 or whatever till it comes up to around the $3 price and that's how many I include in my Multi.

So if I reckon the games are an even money chance,I would not think about taking more than a double.