It's all about knowing your skill level and your gears, then reading the terrain. The gearing is set up to give a certain amount of overlap in the gears from one chainring to the next. You need to know this (by riding the bike, only nerds like me mess with gearing charts) so you get a feel for what the large chainring gives you, what the middle gives you, what the granny gives you.
You'll find that each chainring equates to certain type of riding. Obviously the largest chainring is for high speed work, the granny for climbing mountains. The middle falls somewhere in the ... um ... middle.
With the gearing that used to be on the Europa (she's now an overgeared fixie), a fairly typical 2 at the front, 5 at the rear set up, I'd get rolling and, unless I was looking at hills, would change onto the large chainring fairly quickly, using the smaller chainring only in hilly country.
With my Trek though, with it's very broad set of ratios on the rear (11 - 34) and granny gear, I tend to live on the middle chainring, only changing to the granny on steep climbs (usually before them because that change is a slow one) and only going onto the top chainring if I've got a bit of a downhill run. I can top 35 km/hr on the middle chainring with a normal cadence - if it looks like I'll be holding those sorts of speeds, I'd have changed to the large ring before then.
Because the front derailleur gives you a relatively slow change, you use it to define a type of riding (climbing, general, speed), then use the rear derailleur to fine tune the gears to the specific moment.
It's useful to know where the overlap between the gearing on each chainring occurs. This allows you to shift once ie, just the front derailleur, without having to mess about with shifting at the rear as well.
There are no formulae or rules for how you make the front shift. It depends on you the rider, the terrain and what you are trying to do - a high speed run will be handled differently to just pottering along smelling the roses, and traffic adds its own complications.
Like all things cycling, the answer lies in MORE RIDING. Bummer ain't it
Richard