Ideal top tube length for BMX Regarding BMX bikes, is there an easy way to determine what top tube length is most suitable for a person based on their height? 

In my experience, most LBS only want to talk about BMX size in terms of wheels ("it's a 20 inch" or "it's an 18 inch) since they're mostly considered to be kid's bikes.
This is the tool you're looking for:

https://www.ebicycles.com/bicycle-tools/frame-sizer/bmx-bike

It's not a linear calculation, but it would be fair to say that a rough estimate is: start with a 22.5" top tube for a 6' adult male, and add/subtract half an inch of top tube for every inch of height.