Present con/going to

Hi,

I'd like you to clarify something for me. It's about use of present continous and going to when we are talking about future.

To begin with I'd like to say that [color=blue]we can use them interchangeably when we talk about future actions that have been planned before the moment of speaking[/color] ( I am not sure about it though.)

Exemples :

I am redecorating/ going to redecorate my room on Sunday.

Im meeting Mr. Johnson on Friday , but I am not sure whether the sentence : Im going to meet Mr. Johnson on Friday has the same meaning. Does it?

Prediction - when we are talking about an event in the future basing on evidence visible with the naked eye we use going to AND NOT present continuous.

Exemple :

We see clouds gathering in the sky so we can say " Damn , it's going to rain ! We'd better go home"

Tell me if my reasoning is good

Best wishes