This is all according to the English rules of "Becoming a Believer" on page 179 of the 7th edition of the Call of Cthulhu rule book:
If the character chooses not to believe what was read, then the knowledge of Cthulhu Mythos granted by the book is added to the investigator's skill and the maximum Sanity points are reduced by the corresponding amount, but no points of Sanity are lost. […] However, by finding evidence of the Mythos first-hand, the researcher will realize that those cursed books contained the truth. At that point, the researcher becomes a believer and immediately loses points of sanity equal to his Cthulhu Mythos score.
If Harvey-the-Unbeliever reads Cultes des Ghouls, he will gain + 4% of Cthulhu Mythos and have a maximum sanity of 95. If he reads the Latin Necronomicon (2d10 SAN, + 5 / + 11% of Cthulhu Mythos), he has a Cthulhu Mythos of 9% and 90 Sanity of 90. If he later found a Byakhee and lost 3 Sanity, he would immediately lose another 9 for a total of 12, leaving him in 78 Sanity of 90.
Whether being a believer or not a believer is "optimal" may not be at the point of the Call of Cthulhu, but it should be noted that sanity may be lost for reasons unrelated to Mythos, such as gruesome corpses, the violent death of a good Friend , or being confronted with a phobia, it would be very rare for anyone, except a newly created researcher, to have complete sanity. Also, it is likely that an unbeliever can go longer without preventing Sanity from seeing Mythos creatures, since it would be easier for them to pass their Sanity controls.