coffeealex Yep. All correct. 80 mg/l KHCO3 will get you 40 KH and 120 mg/l will get you 60 KH. 100 mg/l is 50 KH. All are fine.
“my” concentrate is 1.60 grams of KHCO3 in 400 ml water, then use 20 g (or ml) of this per litre of water to get 40 KH. The 400 ml is because I had a jar with a screw top that would hold that much, so scale it for whatever you are going to use - don’t make too much of it, or you’ll need months to use it (and probably end up throwing some away).
If you want to stick to 100 mg/l (50 KH), then use 2.00 grams in 400 ml of water, then use 20 g (ml) of concentrate per litre.