What is the recommended ratio to keep my intake valve open?

This can vary, dependent upon the particular set-up application. If the valve controls flow between the skimmer and main drains, the most common ratio is roughly 50/50 -- keeping in mind that pools with more floating debris may prefer a slight advantage toward the skimmer.

If the valve is controlling flow between the skimmer and a separate suction port in the wall, the ratio should be determined by the flow required for the cleaner or vacuum connected to the wall port -- remembering to follow manufacturer's recommendations for the cleaner or vacuum for optimal flow/suction.

If the valve determines the suction between the pool vs. the spa, then the ratio should be aligned with the pool/spa's installer -- making sure to not overly adjust and cause the water level in the spa to drain, or create overflow of the spa with too little adjustment (unless an intentional spillway between the pool and spa has been installed).