you want to be really exact as to how much water is in your tank? complicated masyado ya. you have to measure the outer LWH ng tank mo then sa bawat face ng tank, subtract the thickness of the glass that you used. tapos saka mo gamitan nung formula above. do the same with your tank. then you have to add the volume of water that would be in the pipes, skimmer, reactor/s .... then subtract mo nanaman yung water displaced by your LR.
its complicated but if you want pinpoint accuracy with your doses, then ...