On the slow down the ending thing.... I go much more drastic.... and over a shorter distance.

If you don't go drastic enough, the effect is that you rush to the ending rather than approach it with grace and confidence. Drag it out with too many measures and it takes on a whole different feel...one of dragging rather than ending....

I have found that a few bpm decrease in tempo is barely noticeable.

IIRC.... my song Dust on the Floor has a slow down that is done over 6 to 8 beats at most....and like Floyd said, that's divided into measures that are 2 beats each and each successive one is 6 to 8 bpm slower than the last. Sometimes even more than that. I'd have to look at the project to confirm the details but I think that's what I have in it.

You can find my music at:
www.herbhartley.com
Add nothing that adds nothing to the music.
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The magic you are looking for is in the work you are avoiding.