We have decalcification machine. Should I add Calmag right from the start? How much? (Autoflower in mostly Biobizz Allmix)

Allmix is bit more fertilized than you need from start, lightmix is for that. I use biobizz calmag after fist week following bottle dosages. Do keep good eye on ph, biobizz is very finnicky with it.

Using soft water? Should be fine for any single grow cycle, but don't use it with mother plants that stay in the same pot for long periods of time. Youd get a Na buildup of concern at some point, but never in 3-5 months of a typical grow cycle. Mg and Ca - two unique elements - are both needed at all times. it should be part of your soil's amendments or in your fertilizer. Between your substrate and your fertilizer you need to provide a well-balanced diet at all times -- not a mad scientist adding this or that at "opportune" times. It's complicated but nothing like that. See "Mulder's chart" for the one of the real causes of its complicated nature. Ratio, pH and overall concentration are equally important to proper fertilization. Some formulas may require more or less Ca because of the realtionships outlined in Mulder's chart. Your target pH can do the same thing, especially if it's toward low end you'll need more Ca than somoene a few ticks higher (low pH will lockout Ca before anything else). biobizz light mix and such has some fertilizer in it.. you need to supplement it, so it makes an exact answer difficult to give due to the unknowns of what is in the soil. I can tell you that when i switched from "hard" water (300ppm) to "soft" water I didn't have to change my dose of Ca or Mg, and my hard water definitely added a good chunk. I'm in soilless, so there's not 'unknown' amount added from my substrate. So, if the instructions are solid, which is never a gaurantee, then stick to that... observe and adjust formula of fertilizer based on plant growth and symptoms. same as always. Another suggestion -- use smaller pots for small plants. your substrate will either stay wet for long periods of time (stagnation is bad) or you have to do some esoteric watering methods that either train superficial roots or again.. stays wet for entirely too long and elevates risks of pathogens and pests. watering should be easy in an approriate sized pot -- water entire substrate, allow top layer to dry, and repeat. you use the volume necessary to get it done and not some volume of water from your imagination or potential OCD behaviours. With a huge pot you have to attempt to water a small column around the plant and slowly increase that watering area as it grows. this is not easy to do and risks all sorts of issues with nutrient content around inconsistent edges of the ebb and flow of moisture. plus, if you don't water all the way down to the bottom, you train superfical roots, which is not the goal whatsoever.