Personally I would have an elevated reservoir to hold RO/DI water and plumb it to the float valve. That way you can gravity feed the float valve. You would only have to make sure that the reservoir is full every so often rather than having to replenish the sump every day.
If you connect your RO/DI unit to the float valve directly, you will not be providing your sump with pure water like you think you will.
Yes your RO/DI unit may be capable of producing 0 TDS water, but when a RO/DI unit first kicks on it puts out a much higher TDS (higher parts per million of dissolved solids) in the water for about the first 1/2 gal or so. Since your float valve will activate your RO/DI unit several times a day ... you will be receiving a higher dose of dirtier water than you would if the RO/DI unit were running for an hr or so. This is called 'TDS Creep'.
If you filled a smaller reservoir of 0 TDS RO/DI water to gravity feed your float valve, ever time your float valve demanded water ... it would only put 0 TDS water in your sump.
If you keep the output of your RO/DI unit connected directly to your float valve for your auto top off, every time the float valve demands some water; it will dose the sump with much higher TDS water.
