You just asked the big question of exactly what a copyright owner and a court would spend time on - it's the great "Grey Area."
The answer is: It might be.
The bigger questions are: How similar? Would a copyright/trademark holder consider it similar? If you won in a court case be good enough for you? Or, do you even have the desire or money to defend yourself in court to win or lose?
There's always a possibility that inspiration can lead to a work that does not infringe. However, it's not about that - there's no measurement that can give a blanket answer. It's all about context and if the holder feels otherwise.
THEN... to make matters more complicated - how you promote, advertise, market, and display that art could influence things.
Going to court because a claim of infringement is a process. It's not about taking a calculator and deciding if it's safe or not. It's about convincing others with the use of attorneys.
So, how similar will be up to you. If it's dissimilar enough is something you can influence but there's no single answer.
What you see in the marketplace and whether they, to your eye, look "better" than the original (assuming you're seeing the original somewhere) is absolutely irrelevant. It's nothing to go by and base your risk on.
"Inspired" artwork, isn't really a level of definable defense if the original artist decides he/she thinks you crossed a line.
Artists everywhere walk this line safely all the time.
Artists everywhere sometimes get sued.
Every single case is different and your decisions now influence the risks you might take as to HOW safe you might be. The trick is to not get to the point where you get taken to court the first time.
So, when you say "if I took a person's design artwork" or "the idea is similar" are your opportunities now to be very careful. There's no mathematical equation that can calculate when you're safely out of trouble of infringement.
If you search this forum and read you'll find many conversations about this. Many are about what's called "Risk Assessment" - generally the more similar the art or marketing the greater your risk goes up.
........................blipfish........................
I am Blipfish.