Yes, you can often do this substitution and I've done it successfully many times when I've run out of milk unexpectedly, but you should know that the substitution is never going to be perfect.
On a purely mathematical basis, let's say your heavy cream is 36% M.F. You need partially-skimmed 2% milk for your recipe. If you go purely by weight (which is almost 1:1 for volume when talking about mostly water), then 250 mL of 2% milk will contain about 5 g of milk fat. To get that from 36% cream, you only need about 14 mL, or approximately 3 teaspoons of cream. (Note - don't do this, read on!)
Of course, diluting cream at a ratio of 15:1 is ridiculous and you'll end up with something that's just really watery, and that's because milk is more than just water and fat. I only provide the above calculation for illustrative purposes, to show why there's no hard rule or even rule of thumb for this particular substitution.
With normal cream, most people go with half water, half cream, or if they're trying to approximate skim milk then maybe 1 part cream to 2 parts water. If you have double cream, I would probably use 1 part cream to 3 or 4 parts water; that will give you a reasonable approximation of the consistency of whole or skim milk without making it too watery.
Please keep in mind that different recipes/preparations use milk for different reasons, so this isn't going to work everywhere. I certainly wouldn't do it in baking, but for many stovetop recipes, the main purpose of the milk is just moisture, so you're fine to substitute cream + water and it doesn't really matter if you're "exact" - which you can't be anyway.
Coconut flour is what remains1 when extracting the milk from coconut. So I doubt you can make anything resembling coconut milk (much less coconut cream) from it, unless the manufacturer has done an exceptionally poor job of doing so already!
Best Answer
Short answer: you can let coconut milk separate and skim it to get a little coconut cream, but can't convert coconut cream back to coconut milk. Diluting the cream just produces something runny and disgusting.
Long answer: Coconut milk is made by grating coconut and running very hot water through it to extract oils and flavor, then straining out the coconut pieces. It is a mix of water soluble parts and fats, and can emulsify just like normal milk or thicken sauces similarly to milk cream.
If you allow coconut milk to sit, the fatty part will rise to the top. This part is skimmed off to make coconut cream, which is much richer in fats. It cooks very differently because of the higher fat content, and does not emulsify or thicken well; however it is quite delicious!
If you're looking to substitute for coconut milk, but only have coconut cream, the best substitution will probably be a little coconut cream + a lot of heavy (milk) cream.
Note: Some people refer to the liquid contained in fresh coconuts as coconut milk, but it is more accurately called coconut water. This confusion has ruined many a recipe!