I find it to be an amazing oversight, either in my search capabilities or in the lack of information available on the best project length to fill a BD-R disc from FCP X - by David R. Beebe
Here is a quick note to provide a little info that I could not find elsewhere. When using FCP X to create a video project to BD-R, a 1 hour duration will not fill the disc. A recent 57 minute project, when written to a 25GB BD-R, came in at 9GB and took 2.3 hours to render directly from FCP X to an LG BD-R burner on 4X media. The results were stunningly good, much better and much faster than previous burns to BD-R using FCE and Toast.

Based on these results, I went back and imported finished videos from FCE in AIC format. The finished BD-R was much better compared to the DVD-R and BD-R created by Toast from the same source. 3 videos were stacked end to end on the FCPX timeline totaling 1:17:00;00. The resulting BD-R is 12.86GB. I also took the opportunity to save the final edit in ProRes422 instead of AIC which is 83GB.