Welcome to PF!!

The way [i]we[/u] design our spillover spas is to run with no spa suction when in spillover mode. In theory, the spilling over acts like a skimmer and sends surface debris into the pool to be claimed by the pool's skimmers. The only time we open the spa drains is when the spa needs cleaning (debris can be swept into the spa drains for filtering) or we manually vacuum the spa seats and floor.

Due to the vaguerities (sp?) of each system set-up - how could you possibly be able to select "that perfect valve setting" to enable you to be able to insure that the spa won't eventually fail to overflow?

However, if it's worked for you in the past, I can only suggest that you don't ever let the skimmer baskets/ MD become congested enough to block any suction through them.