I've used MG before. For vegging I used the green liquid stuff (i.e. houseplant food). For flower you can use the powdered stuff (recommended for tomatoes etc.)

If you look at the packs of any fertilizer you will see the N-P-K values. For veg make sure the N is higher, and for flower the P should be higher.

MG has trace elements which are useful but I found I had to use epsom salts during veg, like every couple of weeks.

I now use a dirt cheap fertilizer called 'chempak' which has all trace elements for a complete grow. It comes in a wide variety of N-P-K values, I use no.2, no.4 and no.8 and mix 'em up a bit depending on growing phase.
http://www.chempak.co.uk/product1.html < check out those prices for nearly a Kg! lasts forever.

If you got the cash tho' get some ferts that are designed for cannabis, like Bio Bizz, Canna etc.