My favorite part about competing Paragon calculators is being able to click the very important nodes, regardless of order, and then find an optimal path. It would be great if the Wowhead calculator let you do that, by not restricting you to only clicking "eligible" nodes.
The fact we cannot set the level of Glyphs is a missing feature that needs to be set. This impacts both the value of a Glyph, and the eligibility of it's secondary bonus, as Level 15 specifically increases the range the Glyph can account for.
Your calculator layout needs a redesign. Literally the top 45% of my screen is filled with unnecessarily large menus and titles. Hitting F11 for Fullscreen only provides marginal benefit. It has a very awkward and cramped feeling to it. As it stands there is no reason to use your calculator over the multiple competing sites.