I’m making a gravy recipe that calls for whole milk. I have two percent, and heavy cream.
What proportions will approximate whole milk? Half of each kind?
tip-tapping away by phone, but why would you care?
Whole milk is about 3.5% fat. So use your 2% and add a little splash of the cream if you must. For gravy, I estimate it will make about 1.5% difference 
That’s heavy whipping cream, specifically, although the carton is does not list milkfat percentage. Most unhelpful.
tip-tapping away by phone, but why would you care?
Well, after some googling i found this:
… which indicates that I need 83% two percent and 17% half and half.
In other words, a splash. Like you said. 
tip-tapping away by phone, but why would you care?
The percentage isn’t critical, you can sort of eyeball it and it will be fine. Half & Half should be luxurious.
I’ve even done the same thing to make milk for cereal and it didn’t matter too much. My dad bought me half and half for my visit, not knowing that I no longer took cream in my tea, but he also uses fat-free milk, which was serendipitous because I could then pour the fat-free milk along with an eyeballed amount of half and half to produce 2%-ish milk in my cereal.
Half and half is just fine on cereal, and should sub for any recipe calling for milk, basically. 2% milk is just wrong.
A few weeks ago, I wrote a Python script to help with this sort of thing. It suggests using 1 part of heavy cream to 24 parts of 2% milk. That’s a lot less cream than I would have guessed, so I double checked with the USDA nutrient database.
.04 cup of heavy whipping cream has 3.43 grams of fat
.96 cup of 2% milk has 4.64 grams of fat
so one cup of the mixture has 8.07 grams of fat
A cup of whole milk (3.25 percent milkfat), has 7.93 grams of fat. So my script was right, within about 2% error.