This is a pregen alternative for when other pregen mods/plugins may just not fit your needs.
You can give this mod a try if other pregens left you with a boot/kill-loop, I personally use this with Lithosphere which is too heavy for my 5gb ram server to pregen using other mods.
After first boot a file called /world/yacp/task.json should be generated.
Stop the server to edit it (in case you don't, it does get overriden on stopping the server).
The Start and End coordinates are in chunk units (not block units!).
Start should always be a lower value than End!
If you want to start a new task, set the value of At to the same as Start and change Complete to false.
Processed and Existing are just last session's stats.
After the pregen and afterwards enabling bluemap, I noticed some chunks have been missing (black spots that wouldn't become a map preview).
Solution: remove all .yacp cache files and run the pregen again.
It will be far faster this time because most chunks exists already and server load should be minimal.