my personal experience is the same as that of bro alpha
last month, my mg dropped to 700ppm because i've not checked for months
at 700ppm, the kh fell extremely fast, from 10 to 6 in just one week as opposed to the normal 10 to 8 for my tank so i can say that allowing mg to be around 800ppm is definitely not good
as i boosted the Mg, i discovered that once the Mg reaches about 1100ppm, kh no longer drops fast.
if you're gonna allow Mg to drop to 200ppm only, MM, i think your tank will crash because kh will drop and that'll cause ph fluctuations