A question about your statement "if your last upvote was a long time ago, your voting power should be 100%". How much is "a long time ago" in days ?

It depends on your voting power the last time you upvoted.
If you don't upvote, your voting power increases by 20% each day.
If your voting power was 50%, it will become 100% after 2.5 days if you don't upvote during this period.
if your voting power was 20%, it will become 100% after 4 days.

Also a little error here: "your vote will be 25% (50% x 50%) 0f $0.40, that is $0.40"

Indeed. It is too late now to correct the error.