How to create a calculate metric that shows month-specific targets

So I believe what you are saying is you want a monthly specific target that you compare your actual with (something like metric - target / metric  OR  target - metric / metric but with monthly specific targets)?

You can create a series of nested IF statements to check for the month, the set a target using a static value:

Then you can built this nested setup in conjunction with your calculation. You will notice that I used occurrences here, this is just the most all-inclusive metric to ensure that there is data to match against the month of the year... Since we are overriding the value with a static target anyway, this can be used for any other type of these nested IF value type of logic blocks.

You will use your actual metric into the math where you calculate the difference and then do your division to get your % difference.