The Ptil Tekhelet Organization makes two types of strings, known as “Raavad” and “Rambam.” The reason is because there is no consensus in the Rishonim as to how many of the strings should be blue and how many should be white. The Raavad strings are all blue, whereas the Rambam strings are half blue. This is explained further here. Also, see the last two responses here.
Here’s a question asked about understanding the Rambam’s approach: “I would naturally purchase Rambam strings to go along with the Rambam method, yet I have hesitations. I know there is technically no problem with getting Ra’avad strings and tying Rambam, yet it seems that it would be ideal to keep a steady method. Yet the number of tekhelet strings (to my understanding) is more severe in halakhic status, and I do not feel comfortable using the Rambam’s method because I do not understand how it fits. I see his point about how the ptil should be placed on the other white strings, yet the verse can be easily interpreted in other ways, and by the use of the word ptil, it seems to imply one full string, end to end.”
Rabbi Mois Navon replied: “Ptil can mean strand or tassel, and so half a string which appears as a full string when folded through the hole of the begged can certainly fulfill the pshat of ptil. Rabbi Dov Lior (Kiryat Arba) and Rabbi Rabinovitch (Maale Adumim) explain that when there is not a mesorah of a psak then we should follow the Rambam.”
Based on my understanding of halacha, if you go ahead and tie Sephardic windings, it makes most sense to use Rambam tekhelet strings so that you get seven white strings and one blue string hanging down from each corner. Of all of the Rishonim and Achronim who explicitly discuss how to tie tzitzit with techelet, the Rambam is generally considered the most authoritative.
