Doing some research, I discovered that the spontaneous vs planned rule for "going to" and "will" is taught to ESL students, but not used otherwise.  I did discover a few good rules of thumb that are decent guidelines and make sense to me (a native English speaker):


"Going to" is a kind of present tense, so use "going to" in situations where the 
present is connected to the future.
Example:
      I feel a drop - I bet it's going to rain.
      I am going to walk to school because my bike has a flat tire.
Use "will" in writing and "going to" when speaking.
Use "will" when talking about the not-immediate future and "going to" for the immediate future.
Use "will" and "going to" interchangeably for making predictions.


If you are looking for a good explanation about when to use either form, I suggest you read this article, which explains how to teach the differences between "going to" and "will".

"going to" vs "will"
Is there a rule, or rules, for how to pronounce ough?

trough  (short o,  gh pronounced f),
thought (short o,  silent gh),
though  (long o,   silent gh),
through (oo,          silent gh),
tough   (short u,  gh pronounced f).
