I've never done a lager. But partigyling is easy. Calculating what you will end up with can be tricky, depending on how precise you want to be. I have only done it with a batch sparge process, though. That automatically splits the runnings into gyles. Two for me, usually. In my experience, with all gyles having equal volumes, the gravity of the first will be slightly more than that of the second, which will be slightly more than that of the third, if you do a third. I also find I get about 3-4% higher efficiency doing 3 rather than 2. I also find that the actual split seems to vary about 3 percentage points, randomly, probably due to poor volume measurements.
I use a spreadsheet to track total mash volume and its liquid component to make my volumes work out. I am doing infusions--never done a decoction, but it seems like it should still work the same way. Remember to do separate hop calculations. Good luck. Getting a free batch of smaller beer is awesome.