What if you do a bit of gypsum and a bit of CaCl2?If I add both but keep Sulfate and Cl in line with profile, that leaves me with 10.3ppm Ca which fits the profile (12) but is nowhere near 20-40 ppm. It's where I started until I re-read Martin's suggestion and I'm paraphrasing here, of 20 minimum for fine tasting lagers and 40 to ensure oxalate precipitation
This kind of takes me (newbie lager brewer) back to the question of which aspect is more important: Ca, sulfate, Cl, Lactic, Bicarb, etc? Like I said, I know I'm likely over thinking and should just follow "Amber Malty" which is easy to hit with my RO water, I just don't know if I will get everything I can from A Märzen with this profile.
Any thoughts to help me clear my head? To paraphrase another member here and the instructor at my LHBS "Making beer is not rocket science, its much more important!"
What is most important is mash pH, by far. Keep your other additions on the low side and ride it out. Odds are you'll be fine regardless.