optimal strobe distance for family portraits?

Hi all,

I plan to do some family portraits for my son's preschool.  I will be using my D700 and 2 SB-800's on stands with 42" umbrellas.  I plan on shooting in manual mode so that I can keep my shutter speed fairly high (I was thinking around 1/250 because kids move around alot) but also keep my aperature at f8 or so to ensure good depth of field.  I plan to set the flashes to manual and play around with the output ahead of time so that I have a consistant flash.  I am not sure, however, where to place the strobes.  Are there any suggestions for an optimal distance I should place the strobes from the subjects that will give pleasing light but not too far away that I won't suck too much power from the batteries?

Thanks!