It is really hard to say.

The ratio is not cast in stone. Magnesium is more likely lost thru the urine than calcium. Diuretics, alcohol, caffeine etc.

Magnesium actually has a very close link with potassium as well.

I don't think taking 400mg of magnesium (I assume elemental, and not oxide?) a day is going to upset much.
In our past as hunter gatherers, we consumed alot of nuts and they are high in magnesium. And much less calcium... not much milk in those days.

I would expect the ratio to be much more important for situations where severe changes or severe differences in consumption were likely.

http://www.ncbi.nlm.nih.gov/pubmed/3732091

And this you will find to be very interesting... explains in good easy terms the inter-relationships:
http://www.jigsawhealth.com/resource...m_medium=email

Just want to add on VitD3 -- this is a highly recommended supplement among patients with lung problems (consistent to what Mrs. D's advice). Personally, I take 4000mg from the supplements. Others take as much as 10000mg and higher everyday.