Own work. Article 14 of the São Tomé and Príncipe constitution contains a short description of the flag but it fails to provide the complete geometry.

The National Flag consists of three horizontal bars, being green and those of the extremes of equal width, and the middle, in which are affixed two five-pointed black stars, yellow, being one and one-half times larger than each of the others and by a scarlet triangle, whose base is located on the left side of the Flag. The height of the triangle is half that of the base.

—  Constitution Article 14 Section 1

The flag law does not provides specifications for flag's aspect ratio. Neither does it provide specifications for the size and position of the stars.

This Wikimedia construction sheet is based entirely on the flag image found in the 5th edition of Complete Flags of the World ^[1]. The book was created in association with the Flag Institute and is a reputable source. A second image with identical geometry to the one in the book can be found at Flying Colours Flagmakers.

Note that on this construction sheet (and on the source material listed above) the vertical distance between the upper green bar and the upper tips of the stars is equal to the vertical distance between the lower green bar and the lower tips of the stars. This gives the stars the appearance of being vertically centered within the yellow stripe. Most international flag makers show (on thier websites) images of the São Tomé and Príncipe flag with the stars vertically centered this way.