How do I scale up to semi-prep from an analytical HPLC
Best practice is to try and maintain the same column length between scales.  Then the scale up factor is simply the ratio of the square of the radii.

If the lengths are different,  try and keep the aspect ratio in the same ballpark e.g. if the length to diameter ratio is not the same but between 4 to 5 for both, you're in a better place. Then simply scale by the ratio of the two column volumes, using the radii calculated from the internal diameters.

Using the ratio of column volumes should work in other circumstances but scaling from a long, thin column to a pancake may cause some issues.

Make sure to keep your particle size of your stationary phase the same too so you have similar packing.
All things being scaled appropriately,  the back pressure should be the same too.