25.5 mpg?

Actually, with all respect to the esteemed authors of the five stages document, it is not strictly temperature-driven. In fact, without an EBH preheat it seems mainly a function of time. Without preheating it's pretty consistently 1+ minute regardless of ambient and coolant temps. I've seen S2 onset in the winter with coolant temp below 80F, whereas in the summer it may be over 120F. The constant is time. On the other hand, with EBH preheating it is indeed as little as 30 seconds, so obviously temperature does play a role somehow. But it is more complex (and mysterious) than just coolant temp.

As for whether letting it idle during S1 is good, here's my approach. If I can drive that first minute or so under relatively light load, I'll drive immediately. If not -- if I need to immediately ascend a steep hill or get into heavy traffic at speed -- I'll let it sit until S2 shutdown. Again, with the EBH, that may be as little as 30 seconds.