In sampling you know the total population being examined, here you do not. You cannot solve for x (total species known and unknown) when all you know is y (known species). You have insufficient data to make a valid determination - as I have been saying from the beginning. There is no criteria to prove that the existing species comprise 1%, 5%, 10%, 20% or even 50% of the total amount of species ever. Such a ration could only be arrived at if we know the total number of unknown species in the first place which by definition we do not know. It's garbage.
No, it is most definitely not garbage. You don't always know what your total population is. You don't need to. This is part of everyday science and experimental design.
Do yourself a favour and read up on statistics.