Here is a link to the webinar @john.kempf did a year or so ago on designing foliars. I found it very helpful. https://www.youtube.com/watch?v=JWvLjSpHAKs

AEA’s product usage guide, found on their website, also has some tips regarding foliar sprays.

As far as application before rain, for foliar sprays you would not want to apply within 2 hours of rain because it would wash the foliar spray off before plant absorption. Now if you are applying a soil drench or soil primer it could be good to spray right before a rain as the rain would soak the products into the ground.