General strategy: You gather power (large) for a big opening strike and then you gather power (small) so you can do it within a round and attack (instead of gathering for a full round). That will be enough for most encounters.
You want to throw a blast per round without burn, so you adjust infusions and metamagic so that Gather power comes up to 0.
For the opening strike, you can do more metamagic/infusions with Large Gather power.
Generally, you want to accrue just enough burn for the first stat boost and not more. However, during boss fights, you may want to throw a metamagic or composite blast right away or after moving and getting into range. Or you might want to make your opening blast much stronger to deal with a troublesome encounter with more burn than Large Gather power can erase. Or simply enough., yo uwant to use one of the utility powers (like throwing a persistent slick for example, which is like grease) This is acceptable, but you should not waste burn unless it's necessary.
You can easily have Empower+Basic blast at 0 burn cost with small gather power.
As you get stronger, burn cost for metamagic and composite blast will drop, so you will be able to have an even stronger blast on auto-attack.