When I did a brief stint in dosing, I used one of the units form bulkreefsupply.
Before we go that far, can I ask why we are dosing? This is a very small system that can easily be managed by water changes. You will want to calculate the total water column, so yes include the sump, but it takes a very heavily SPS
stocked tank to require dosing to replenish the elements that are being consumed.