In todays world, 71% of the surface is covered in water. But was this always the case, far prehistoric times? Id guess you can estimate pretty precisely where the prehistoric oceans were based on sediments, but i cant find a single answer to the question - even though you can see detailed maps of Laurassia, Gondwana and Pangaea.
... because of the significant tectonic activity occurring around the world, it is not clear which of these local changes can be correlated to global sea level change. Because there is no evidence of major glaciations in the Jurassic, any global sea level change must have been due to thermal expansion of seawater or plate tectonic activity (such as major activity at seafloor ridges). Some geologists have proposed that average sea levels increased from Early to Late Jurassic time.
Notice, that we have no information about the change of the mass of water from Jurassic till now.
Even the articles https://www.sciencedirect.com/science/article/pii/S2095383615000267 that shows, how the sea level changed, touch only relative changes in the sea level. As we don't know the statistics for depths and heights for these times, we can guess, that the ocean/land area ratio was greater than now, but the guess is very wild. Any number of percents could be only a sheer speculation.