Your spine width is the one cover dimension you cannot estimate. It is decided entirely by how many interior pages you have and which paper you print on, and a cover built around the wrong number will have its title creeping onto the front panel or its artwork sliding round the back.
This calculator multiplies your page count by the per-page thickness of the paper stock you choose, then adds that spine to two trim-width panels and the bleed on each outer edge to give you the full cover size to design against.
Paper choice moves the number more than most people expect. At 200 pages the same book measures 0.4504 inches on white and 0.5000 inches on cream, which is enough to misalign a spine title that was designed against the wrong stock.
The calculation is page count multiplied by the thickness of a single sheet, and the stocks differ. Cream is the thickest of the common options, so the same book is a visibly wider block on cream than on white.
Round up. A spine designed slightly wider keeps your artwork continuous across the fold. A spine designed too narrow leaves a visible strip of the wrong panel on the front or back.
Both. It shows the spine on its own, and the full cover width and height with the front and back panels and the outer bleed included, which is the canvas size you actually design on.