I have in my suggestion reduced the number of sign offs by 5, but replaced those 5 sign offs with the 3 First Solo's and 2 Skill tests.
I put the six months in as it is quite possible you could remove your restriction over course of (should we ever have one again) a good summer where the weather is kind for 2 or 3 months. The sixth months should at least provide the Instructor with an opportunity to have to make some decisions when the weather is at bit more marginal for want of a better phrase.
The twenty sign offs give the opportunity to perhaps come accross the student who for whatever reason despite it maybe being 3rd/4th solo etc, is not ready to go on the day in question. It might be cross wind a bit more then you anticipated, it might be student has something on there mind etc.