You handled the math beautifully. BTW, you don't have to use a 60/40 sponge. You can use, say, 70/30 or some other ratio if you'd like. However, the 60/40 ratio is a good place to start. Also, the sponge conversion method can be used with many other dough recipes.
As for your proposal to hold the sponge for several days, I wouldn't do that. During the period that the sponge is fermenting, the yeast consumes the sugars released from the flour by enzymatic performance. Over a period of several days, it is unlikely that there will be enough natural sugar released to keep the yeast going, and the most likely result is an over-mature and very stinky, acidic concoction with little or no leavening power left. Also, unless the sponge is kept in a sealed container, the water in the sponge will evaporate and leave you with a dry, crusty, highly acidic and stinky concoction with little or no leavening power. You might be able to add it to the rest of the ingredients in order to get more flavor in the finished crust but you would have to add more IDY as part of the final mix. Also, the high level of acids can affect the strength of the dough, making it overly elastic, and other acids formed in the process can create flavors in the finished crust that you may not like. The usual practice with sponges is as described in the article I referenced.
Usually, the way you can tell when the sponge is ready to be used is to watch the surface of the sponge. When the sponge becomes bubbly and cracks start to form and the sponge peaks volumetrically and starts to collapse, the sponge is ready to use. It is possible to extend the fermentation period of the sponge in your case, but you would have to use much lower amounts of yeast. And you would have to time everything such that you are there when the sponge is ready to be used. You are playing with chemistry here and, once made, the sponge and Mother Nature, not you, will control the process.