i have ro/di lines ran throughout the house so i don't need to bring out any hoses to do a top off i just open the valve to the tank i want to fill (lines are secured to the sump) and hit the timer to activate the timer on the ro system than i set a backup timer on my phone that is always on me so no matter where i am i get a reminder. each tank ive got the times down to a science as i always top off at the same mark. when i fill a mix tank up again i know how long it takes to fill and the timer even at its longest time setting wont flood it also the mix tanks are in the unfinished utility room that's equipped with a floor drain. we installed this system after flooding 1/3rds of the house
. i typically dont have a storage container of strait ro/di. your system sounds rather similar to my setup just a top off has be manually started but will automatically stop after the desired amount has been dispensed.