To find out, I made five batches of biscuits that were identical except for the amount of milk used. Each batch uses two cups of flour and normally makes 8 biscuits, but I only cooked two biscuits from each batch so that I could make sure the baking conditions were exactly the same for all of the biscuits.
- Batch 1: 1/2 cup of milk (1:4 ratio)
- Batch 2: 3/4 cup of milk (3:8 ratio)
- Batch 3: 1 cup of milk (1:2 ratio)
- Batch 4: 1-1/4 cups of milk (5:8 ratio)
- Batch 5: 1-1/2 cups of milk (3:4 ratio)
Surprisingly, all of the biscuits were extremely good. I went into this thinking that the amount of liquid ratio was probably the most important variable in the whole recipe. It turns out that it's not. The very dry biscuits were really good, as were the very wet "drop" biscuits. My favorites were probably the ones in the middle (with 1 cup of liquid per 2 cups of flour), but that was by a very narrow margin, and my decision may have been influenced by the fact that a 1:2 ratio is easy to remember.
What about handling the dough?
Again, the 1:2 liquid:flour ratio was about the easiest dough to handle. It was slightly sticky, so you could save some mess by going with a 3:8 liquid:flour ratio. The very dry biscuits were difficult to get mixed, and the very wet biscuits were, well, very wet.
1 cup of milk (or water, or buttermilk) per 2 cups of flour is a pretty good ratio, but it's really not that important.
This experiment also (mostly) debunks the theory that you have to make your biscuits by using a scale rather than using cups and teaspoons. Yes, it's more exact to measure the actual mass (or weight) of your ingredients than it is to measure volume, but this experiment shows that huge variations in the amount of flour per liquid can still result in good biscuits. The small variation in flour amount you'll get by measuring flour by volume will not ruin your liquid to flour ratio enough to ruin your biscuits.
Side note on the Side note: I say mostly debunks because, theoretically, some other ratio could be super crucial to the science of biscuit making. We've already seen that the baking powder : flour ratio and the liquid:flour ratio have a lot of leeway, though, so unless the fat : flour ratio needs to be honed to perfection, a small four measurement error just isn't going to make much of a difference.