For what it's worth my simple rules of thumb:
i
1. If it's ale wort under 1050 and the pack is a month or two old and swells on the same day - pitch directly or into 2 L of identical wort saved from an NC brew. Pitch when active.
2. If using dry (rarely do), sprinkle straight in
3 If over 1050 or if it's a lager, I definitely make an active starter - strong beers and lagers will get up to 6-8 L from a single, whole smack pack. I no longer split.
Not theoretically best practice by any means and I recommend anyone interested in cell counts and the effect on beer gets reading (plenty of references) and possibly invest in a microscope and start cell counting.
For me, the above approach works well. All my batches are 18-22 L.