I am looking for a textual source for this famous GRA which corrects the מקרא calculation of Pi from 3 to 3.1415 based on the kri and ksiv of קו vs קוה. Based on my research there may not be a written source in the Gra. If not, does anyone know the actual text that suggests this correction?
According to this (p. 9) the Gra never said this, it was actually proposed by Max Munk and was wrongly attributed to the former,
This solution is attributed to Eliyahu the Gaon of Vilna (18th century), known as the Gra. The Gra was not only a Gadol in Torah, but also an accomplished mathematician. However, there is evidence that this solution was first proposed by 20th century Torah scholar, Rabbi Matityahu Hakohen Munk (Max Munk), as noted in a private communication to me by Rabbi Professor Sid Leiman of Brooklyn College. He states: “The Gra did talk about pi, but never suggested the secret interpretation ascribed to him. That interpretation was first suggested by Max Munk in 1939. He published his suggestion in “Shalosh Ba’ayot Handasiyot be-Tanakh uveTalmud.”
(This article was published in the journal "Sinai", by Mosad haRav Kook, Tamuz 5722. Pp. 218 - 227)