How to "Sponge" in "Sponge and Dough"?
There are many ways to make a sponge.  Probably no way is totally wrong (OK, maybe adding 10mL of water to 1000g of flour is totally wrong).

There are wet sponges and dry sponges. Supposedly the ratio of flour to water in your sponge can have different effects on the final product. I haven't paid enough attention or done scientific enough studies to decide whether this is actually true. These two general classes of sponge (wetter and drier) are often called poolish and biga respectively. There's a ton to learn about pre-ferments.  Here's a discussion on the difference between poolish and biga: Funtional effect of biga vs poolish  

So, is your ratio of flour to water wrong? Not necessarily. I'd imagine you can find a recipe for a biga with that exact ratio. I just pulled a cookbook off my shelf (Flour, Water, Salt, Yeast) and found that the ratios in the biga recipes are similar to your recipe: 340g water to 540g flour and 544g water to 800g flour (that's 68% in both cases).  In both cases the biga is a larger percentage of the total dough than in your case (50% and 80% respectively), but that probably doesn't matter too much.