Currently it's somewhere between 50%~60% of fuel tank's volume. From what I've heard, you took this ratio based on Shuttle's LOX tank. The problem is your base was improper: LOX tank of Shuttle is not a cylinder.
Cylindrical tanks should be able to hold at least 99% of it's volume, considering there's no practical internal structures and their wall thickness is practically negligible.
The real internal structure comes from intertank and such. Since we're flying bipropellant rockets, our fuel tank should be consisted of the actual fuel tank and oxidizer tank. Even though 60% utilization is way far small.
A maintainer of KSP Realism Overhaul, Pap, said that 90% utilization would be a good number if people build rockets without intertanks.. Builders like me make all the intertank and such by myself, so for my kind of people the utilization should be 99% or higher. In my opinion, you can make a hidden parameter accessible via tinker panel so that players can set their tanks' utilization.