the volume in hydraulic retention time is of reactor/container or liquid

I have a tiny reactor, and the volume I need is 425ml. If there is no/few other reactants, then the volume in hydraulic retention time is clear: reactor/container’s volume. and hrt is V/u(V is 425ml, u is flow rate) But if I fill 80% of container with something like lime mud, the real volume of water is only 20% left — 85ml. How is my V should be? 425 or 85ml? IOW, the volume in hydraulic retention time is of reactor/container or liquid?