Off hand the only risk a normal person might have is runny stool.

You're dose is moderate. I'd only worry about serious harm if
you took mega amounts of magnesium hydroxide such as formerly
used by some in the face of chronic acid reflux or acid stomach.

Your diet will contain some calcium. People often eat too much
dairy (that includes me) hence too much calcium, IMO.

I would read your nutrition labels and try to ascertain how much
calcium you are actually intaking from your diet. If your supplemental
magnesium is higher than your dietary calcium intake, i would consider
supplementing some calcium as well. I am not sure if you will ever see
runny stools from high dose ionic (chloride form?) magnesium since it
is so well absorbed. From what I have read, diarrhea is more commonly
a side effect of non-ionic high dose magnesium, like the oxide form.

Quite right.  You want to keep them in balance, and each tends to
interfere with the other.  It's really quite complicated, so look into
it in a standard nutrition textbook or the online Merck Manual.

You can't generalize about whether Americans get too much or too little
calcium.  Our diets are too variable.

I don't want to oversimplify, but in general, if you're having trouble
sleeping, taking magnesium won't help, but taking calcium may.  Look up
the deficiency and toxicity symptoms of both.  Magnesium tends to
increase muscle tone and calcium to lower it, but the devil's in the
details.

Look up vitamin D, too.

The milk of magnesia formula I have has magnesium hydroxide.

One teaspoon contains about 525 mg of magnesium.

Three teaspoons is the recommended level for use as
an antacid.

Three to four ~tablespoons~ is the recommended adult
dose as a laxative.

Partly for the antacid effect and partly for the magnesium,
I sometimes take a teaspoon in water along with vitamin
C or aspirin, -when I'm taking them on an empty stomach
which is usually the case as I'm trying to eat a very limited
number of meals.

 Milk of magnesia is incredibly inexpensive- but I do worry
a little bit about the form and all.

 Many years ago I read in the Linus Pauling Institute
protocols for megadosing of C for cancer the diarrhea test,
or some such name. Basically, you increase the dosage
until you get diarrhea and then cut back. I've been kind of
expecting to use that to make sure I don't get too much
magnesium, but apparently I haven't come close to the
laxative effect yet.

Richard

Define ok