Future plans - verb tenses
***Please pretty please ONLY answer if you are a native speaker who knows his grammar thoroughly***
I have a question about verb tenses used in talking about future plans.
I know that for future arranged plans we use the present continuous, for example:
"I am leaving after dinner."
But sometimes we say 'will', in spite of the fact that it is a plan and not a guess of what might happen. For example:
"I am taking a shower, and after that I'll be joining you for dinner."
It seems to me this is correct, as I hear native speakers use this form regularly, but I cannot find it in the grammars. Is it indeed correct? And why? Or is it wrong?
Is it perhaps because I have made a clear plan for myself, but the other person - to whom I am speaking - is being informed of my plan and thus has not technically made an arrangement with me yet? So in a way I am offering them this as a plan - so it would be considered a mixture between a promise ('I will') and a plan (continuous: 'joining')?(By the way, I know this is a future continuous, I am particularly asking about its use in these cases, when plans or arrangements are the topic.)