As to improve user experience, what is the standard line height for a 12 px font size? And what is the standard space between paragraphs for 12 px (font size) body text?
There's no specific rules for how much line-height we should actually implement on the content. This is a bit more a design decision in according to specific requirements of the content. Here are some materials that can really make you able to identify the best suited line-height for your content.
There are a tons of articles helping you out about line spacing :)