Wood moves across the grain as its moisture content follows the seasons. Enter the part width and the two moisture contents it will live between; we use the USDA shrinkage-interpolation method, not a rule of thumb.
Not sure of the moisture contents? Get them from temperature + humidity with the EMC calculator.
Free and unlimited. Checking a whole project at once — every shelf and panel, saved and printable — is the $39 Design-Checked Project Kit.