Jim, it sounds like you have the process dialed-in for your environment. As much as I would like say that culture X pitched into 1L of starter wort is going reach high krausen at time X, yeast behavior just cannot be quantified to that level of precision in a general way, especially when things such as water composition, wort composition, incubation time, and incubation temperature are factored into the equation. That's why I give a window of between 12 and 18 hours after inoculation. The best thing that one can do is what you are doing; namely, inoculating, observing, and adjusting the incubation time frame per one's observations.
In your case, I would feel comfortable making the starter in the morning and pitching 12 hours later. The worst case scenario is that you have to place a chilled batch of wort in your fermentation chamber for a few hours. The one area where Wyeast absolutely blows White Labs out of the water is that one knows something about how the culture will perform before pitching it into starter wort. Wyeast smack packs have built-in proofing. If the pack swells, the culture is good. The rate at which the pack swells also provides insight into the condition of the culture as well as to how well it will respond when pitched into starter wort.