bubba_p wrote:After experimenting, each button seems to deliver various amounts of water.
The
Buyer's Guide to La Valentina covers the semi-automatic model. The only practical difference between it and yours is the volumetric dosing, which is used for ending an extraction after a fixed volume of water has been dispensed. This feature comes in handy in cafes or busy households when the barista suffers a moment of inattention, but most baristas watch the extraction and end it when blonding begins. You may find it useful to program one dosing key to the flush amount from idle and another to slightly more than that needed for a double espresso under the assumption you'll cut it off manually.
In the guide, I refer to the preinfusion of your model (and the semi-auto like mine) as
progressive preinfusion because it relies on the gicleur (orifice) and vibe pump for slower pressure ramp up as compared to a "true" E61:

bubba_p wrote:The first button seems to be just enough to pre-infuse and I have been using it as such. Then, I go ahead and pull a shot using the semi-automatic button. Is this correct form?
I refer to the technique of running the pump for a moment, pausing, then starting again as "pre-wetting" the puck, not preinfusion, because the group depressurizes completely. That's not the purpose of the volumetric dosing. I tried it and found pre-wetting the puck beyond a couple seconds muddied the espresso's taste.