Digital BW, The Print

Hi,

I am intended to buy an old/used densitometer. I have seen a model 
which seems to measure only R,G,B *density*. 

Is there a mean to get a BW *density* value (by calculation) from R,G,B 
density values... I was thinking at a formula like DBW= 0,7*DR + 0,2*DG 
+ 0,1*DB  (values are just for illustration) ?

Thanks !

J.