Consider mashing at around 148 and pulling the decoction at 45 minutes and mashing for 75-90 total time. The grain should provide sufficient calcium.
I was under the impression that I would need to begin the mash at approximately 120F for a few minutes, then pull the decoction to get to another rest temperature. Are you suggesting I start at 148 and then pull? What temp am I aiming for once I add the decoction back to the mash, or is this to get to mash out temperature?
A rest at that low temp with modern malts can be problematic.
Try 148 and see. I’m assuming that Denny’s point is regarding the 120 step. I have had no problems with 148 as a starting point, even lower (Hochkurz step mash starts as low as 143.6). I appreciate that a single infusion is all that is needed, but I frequently mash at 148-152 for a batch sparge even with the best of modern malts. The OP wanted to know about decoction step mashing. The 120 is problematic, I agree - there is no need for Ferulic or protein rests and they can be problematic to the flavor and head on a lighter lager such as this.
To the OP, you can add back the decoction incrementally to hit your second mash step temp, then let the rest of the decoction sit on the side and cool, then add back when it gets to the main mash temp or add it back , if another step is desired. Lotsa ways to skin the cat, including just a single infusion mash. Historically this style was decocted, but I only decoct when I want to do something different for a change.