Halakhah zu Wajikra 13:5

Although vertical poles of a tzurat hapetach need not touch the horizontal pole (or wire), they must be positioned directly underneath it. The poles cannot even be off by the slightest amount (see Mishnah Berurah 362:63). Halachic authorities debate how to determine the proper positioning.²Measurements for some areas of Halachah are estimated based on what appears correct to people, while other areas require precise measurements. For example, terumah (the fiftieth of grain which is given to kohanim) must be an estimate and may not be measured to precisely equal one-fiftieth (Terumot 1:7). On the other hand, techum Shabbat (the area that one may not leave on Shabbat) must be measured precisely (Eruvin 57b, 58b). In many areas, it is unclear whether an estimate or precise measurement is required. For example, the Chazon Ish (Hil. Tumat Tzaraat 8:1) writes that measurements for the spreading of a spot of leprosy are done by estimation. He bases himself on a passage in the Ramban's commentary to the Torah (Vayikra 13:5). However, the Chazon Ish does not mention that the Rosh (Tosafot Harosh, Mo'eid Katan 7a, cited in the Tur's long commentary to Vayikra 13:5) requires the use of measuring implements to determine the leprosy spot's growth. Rav Yosef Dov Soloveitchik (as reported by Rav Yosef Adler) and Rav Moshe Feinstein (reported by Rav Elazar Meyer Teitz, from his uncle, Rav Pesach Rayman) both felt that it is sufficient to estimate the poles' positioning with one's eyes. Rav Zalman Nechemia Goldberg (personal communication) also permits using eyesight, although he requires building very wide vertical beams to allow for a wide margin of error in their positioning.