Simple cell count

Dear colleagues,
Can we count absolute cell number/concentration by simply dividing total
cell count (or number of events collected) by the volume passed through
the flow cytometer? 
The volume here is: 
(volume of counted cell suspension) = (volume in the tube before flow) -
(volume in the tube after flow) - (volume of the dead space the flow
Cytometer lines between the intake and the measuring chamber)
If yes, how can we find/measure that dead space volume?
Best regards,
Sergei