Neither is it possible for Reflect to know that in advance of the actual backup operation.
Reflect must compile a list of files it has to write for the next incremental. It would be a simple matter of adding all the file sizes together. Reflect has access to the file system and can do arithmetic faster than a mere human.
Also, Time Machine on OS X does exactly this. It calculates the space required for the next incremental, deletes the oldest backup files if there is not enough free space, and has had this functionality since 2007. This is not an impossible task.
Speaking only for myself, I sure wouldn't want that case-by-case decision making option to be replaced by some "fully automated" guess even if it were possible.
I'm not asking that the default behavior be changed. I just want options in the backup definition file so that I can automate the choices if I want. If you don't like it, then you aren't required to use it.
if Reflect is in the middle of a backup it would have to somehow save what it was doing, do the incremental merge, then go back and see if can finish the backup with the extra space it created. [...] this would seem to be doing two operations at the same time.
That's why Reflect should calculate the free space required in advance, so that it can merge the incremental(s) first. Once that's done, then it can perform the next incremental backup.