Explore "A Treatise on Grain Stacking" by John N. De Lamater on Textopian. Read with annotations and highlights.
For a pyramid stack, build as usual up to within two or three rounds of where drawing in commences, then draw in a little at center of sides and ends to bring the curves to straight lines; keep the corners well out, observing the form of a rectangle in filling the middle, and finish to top.
For a gothic stack, build an ordinary one until commencing to draw in, then draw in the oval corners and build center of sides and ends straight up. For an X stack draw in sides and ends; build center straight up. These stacks look very ornamental on a premium farm and will save well, but take more time to build than ordinary stack tops.