It is very common in older recipes to specify the consistency that adding flour should produce, rather than an amount of flour. Flour from different companies could be very variable in how much liquid they absorbed. This is still true today, in my experience, when using whole wheat flour. With white flour, however, consistency is very… consistent across brands.
Normally, when updating a recipe like this I will specify the amount of flour needed. In this case, however, the amount is going to depend heavily on how the molasses reacts to being boiled.
Like most ginger snaps, this recipe requires patience. Wait until the snaps are cooled completely through for best flavor and texture.
Don’t make cooking a science. Adapt it as an art. — Vrest Orton (Cooking with Wholegrains)