First of all - MITS (Magic in the Shadows), Page 100 gives the details on how to summon watchers (I've converted the text from a paragraph into a numbered list at each sentence end, to make discussion easier):
- A watcher is a simple spirit that a magician can conjure and give simple tasks to perform.
- Any magician able to use Conjuring to summon spirits can summon watchers.
- Generally, the summoning ritual requires no special equipment and can be performed at any time.
- To summon a watcher, the magician makes a Conjuring Test against a target number equal to the watcher’s Force.
- Every success gives the watcher one hour of life span.
- Drain for summoning a watcher is always (Force + life span hours)L, and is Stun damage, never Physical damage.
- The maximum Force a character can give a watcher is equal to his Magic divided by 2 (round up).
- The summoner must resist conjuring Drain by making a Charisma Test (p. 162, SR3 ).
- An initiate may use Centering to reduce the Drain (see Centering, p. 72).
- The summoner may also choose to shorten the watcher’s life span to decrease Drain.
- A character can maintain a number of watchers at once equal to his Charisma.
- Watchers do not count against the number of elementals a mage can bind at one time.
- If necessary, keep separate track of these two totals.
- Characters can dissolve watchers they have conjured at will, even before its time expires, whether or not the spirit is present.
- Watchers can be summoned for longer times by spending Karma, using ritual materials, or combining the two.
- A magician can conjure a watcher that lasts for weeks, rather than hours, by paying Karma equal to its life span in weeks or expending the same number of units of ritual materials at a cost of 1,000 nuyen per unit.
- For example, a magician summoning a watcher for 5 weeks can pay 5 Karma, use 5 units of ritual materials, or any combination of the two (for example, 3 units and 2 Karma).
- Whether its life span is measured in hours or weeks, a watcher dissolves back into the formless energies of astral space when its time runs out.
- Watcher spirits can only ever have 1 Karma Point.
So, the point in question, for me, is point 10 - reducing the lifespan, to reduce drain.
Example 1- you have Conjuring 5. You want to summon a Force 5 watcher. You roll 5 conjuring dice, and get 2 successes - so you now drain a (Force 5 + 2 successes) 7 L drain for the summoning, and get a Force 5 watcher for 2 hours.
Example 2 - you have Conjuring 5. You want to summon a Force 2 watcher. You roll 5 conjuring dice, and get 5 successes - so you now drain a (Force 2 + 5 successes) *still* 7 L drain for the summoning, and get a Force 2 watcher for 5 hours.
My initial thought / assumption was that if you wanted to reduce the duration, and thus the drain - you would withhold dice, just like you would for spellcasting to reduce the AoE of an effec (SR3 rulebook, Page 181 - you can reduce the AoE of a spell by 1 meter per 2 dice withheld from the sorcery test), or to limit successes. The sentence for me is very woolly, and does not say if there is a limit or mechanism for how you do this. I've checked the FAQ and can't see anything about it either.
My interpretation:
Example 3 - you have Conjuring 5. You want to summon a Force 2 watcher, but really don't want drain.. You roll 2 conjuring dice instead of 5, and get 2 successes - so you now drain a (Force 2 + 2 successes) 4 L drain for the summoning, and get a Force 2 watcher for 2 hours.
My gut call is that if you want to limit the successes, and duration / drain, that reducing the number of dice rolled is the way to go - so it fits with the mechanics of other similar mechanisms, and the rules are consistent - and it's the implementation in game that has the randomness. However, I'd be interested to see how other people interpret that, or would deal with this as a house rule?