There are three opinions among the Rishonim with respect to the ratio of white strings to Tekhelet strings:

• Rambam Half of one string (when folded becomes one of the eight strings) is Tekhelet. The Rambam understands the pasuk in Bemidbar in the following manner: put upon the fringe of each corner (= white) one thread of Tekhelet. Only the windings (ptil) around the white core (k’naf) must be Tekhelet (Hilchot Tzitzit 1;6)
• Raavad and the Aruch, based on the Sifre in Shelach hold that one full string (when folded becomes two of the eight strings) must be Tekhelet (Hasagot, Hilchot Tzitzit 1;6)
• Tosafot-Rashi Two full strings (when folded becomes four of the eight strings) must be Tekhelet (Menachot 39a and 41b “Bet Shammai”)