Mechanics moments question =)

Basically i get the answer but i don't understand it lol:

A mirror of mass 24 is supported by two screws 0.8m apart. One of the screws breaks so someone applies a force F to it, 1.2m below the screws to keep it in the same position.

Find the force required.

I'll draw a picture =)

http://img134.imageshack.us/img134/8063/mechk.jpg

Oh and there's a mass on here which i forgot to draw sorry. Acting from center with force 24g.

Ok, so my working went like this:

1.2F = 0.4 x 24g

And i got the correct answer of 78.4N

Then i managed to confuse myself.

I took moments at the broken screw..but why is one distance taken horizontal (0.4, half of 0.8) whilst the other is vertical (1.2)...shouldn't you take both verticle or both horizontal? i.e (1.2 and 0.6) or (0.8 and 0.4)?

Thanks =)

Hopefully i explained that ok.