Add one foot of head for each foot of rise. Estimate the horizontal distance from the pump to the water feature. For every 10 feet, add one foot of head. On runs less than 20 feet, skip this step. Flow rates will be reduced by using tubing is too small.
Larger tubing allows water flow without friction loss.
Never run a pump dry. Submersible pumps are water cooled and will overheat when operated dry. Flow can be reduced by partially restricting the output of the pump. Restrict the flow of a pump at the top of the waterfall, not at the pump itself. If very large flow rates are required, run two or more pumps in parallel.