if you are converting straight text, Book Designer is a good tool to help you convert to LRF. You'll need to get the Book Designer installer, the update, and the Book Cleaner files.
Book Designer and update:
https://www.mobileread.com/forums/showthread.php?t=11786
Book Cleaner files:
https://www.mobileread.com/forums/showthread.php?t=11786