I'm looking for a specifically lines to pages converter. A Google search didn't really help. What's a formula or resource I could use?
You can't determine the number of lines per page until you first determine the number of words per line—and the online calculators I've seen convert from words to pages, not lines to pages.
Any reasonable calculator should at least take into consideration font face, font size, and line spacing in order to provide a general guideline.
Here are some online "calculators" I found that will let you adjust the font face, size, and number of words in order to produce a page count estimate:
Note that the specifications for the book (border, format, and so on) can change these calculations even further.
You can estimate the number of pages given all of these variables, but if any of the variables change, the number of pages will also change.