Proportionality calculation

Hi Can anybody guide me how the data will be calculated for the following scenario.

let  A be the parent level  which has children b1,b2,b3,b4

Now user performs baseoverride at level A( base override is proportional to baseline forecast).

LEVEL  A:DATE 01-DEC-14 BASEOVERRIDE : 150

VALUES OF BASELINE FORECAST (01-DEC-14)

B1 NULL

B2 0

B3  NULL

B4 0

Now the baseoverride is disaggregating to B2(81.96) , B4 (68.04) (not an equal split)(81.96+68.04=150)

since baseline forecast is refernce series and it contains NULL and zero at lower level ,please explain how the value is getting splitted .

Regards,

Aiswarya.