As @gerrit commented, a precise calculation would need to incorporate a litospheric model to account for isostatic post-glacial rebound. But the "blurry approximation" you want, can be obtained based only on topographic data. This approximation would be reasonably accurate if the melting of the ice happens quickly.
For this, we need topographic data of the current surface, and of the bedrock topography. That's the bare ground below the ice. The only source of such data at a global scale (at least that I'm aware of) is ETOPO1, at a resolution of 1 arc-minute (roughly 1.8 km).
A visualization of current surface data from ETOPO1, using an equal-area projection (Eckert IV) and setting the sea level at 0 m looks like this:
And if we measure sea and land areas, it says that currently, a 70.97% of the global surface corresponds to oceans, pretty consistent with the figure you have. Be aware that this simple approach does not consider lakes or rivers, only oceans. It would also misinterpret below sea level dry land as ocean. However, all those factors are arguably minor contributions at a global scale.
Now, if we use the bedrock version of ETOPO1, and set the sea level at 66.5 m, consistent with the best estimates of water stored in land ice. The globe would look like this
In this case, if all the ice melts, the fraction of the Earth's surface covered by water would be 75.18%. Equivalent to an increase of 4.21% relative to the current value.