ATM effective balancing weight for a dob

["Arjan te Marvelde (ELN)" <Arjan.te.Marvelde@eln.ericsson.se>]

What is the most effective place on the bottom of the mirror box to put the balancing weights?
If you consider a mirrorbox with weights in the two bottom corners on both sides of the alt axis, what would then be the equivalent weight if only put in the center of the box?
 
My gut feeling is that it is just the sum of these two original weights, but i don't trust my guts lately (and i'm too lazy to calculate it myself ;). 
Possibly i overlook the fact that the center of the bottom is closer to the axis than the corners, otoh this effect cancels out between corners.

Thanks,
 ... Arjan te Marvelde