I want my RO/DI unit to fill my reservoir automatically, but not over fill it.

Program which uses 2 float switches to fill RO/DI reserve:

The program below uses a "dummy" timer.  A dummy timer is one which is configured in the AC but is not associated with a real address (e.g., don't assign it to a DC8 or other 'real' device); I traditionally use addresses out of range for the direct connect and other Neptune accessories (e.g., N01-N16, P01-P16)

// Init dummy timer to off
If Time > 00:00 Then DM1 OFF       

// If lower switch closes then turn on dummy timer
If Switch1 Closed Then DM1 ON

// Allow the timer to stay on long enough to fill holding tank
Max Change 030 M Then DM1 ON

// RO/DI defaults to OFF
If Time > 00:00 Then ROD OFF

// If dummy timer is on (because low water level hit) then turn on RO/DI
If Timer DM1 = ON Then ROD ON

// If high water float hit then turn off RO/DI
If Switch2 Closed Then ROD OFF

The value indicated in the Max Change statement should be large enough to allow the holding tank to fill + a little. The Max Change gives you a backup in case Switch2 fails. If the holding tank fills before Max Change time then switch2 will float and turn off RO/DI but if there is a failure in Switch2 then Max Change will expire and turn off the RO/DI.