The real reason why Stalagmites -> Chuck is better than Chuck -> Stalagmites.
As you know, Stalagmites' stun/damage instance impacts every 0.25s & it impacts 4 times, meaning that the total timeframe to get all 4 Stalagmite stun/damage instances in is 1 second. However, there is a 0.2s delay before the first Stalagmite instance actually impacts once the projectile Stalagmites actually reaches its destination.
From the code, Pebbles' Chuck projectile has a lifetime of 1 second. Now you may be thinking "Well, how on earth is it actually possible to get Chuck's projectile time to include all 4 Stalagmite instances if Chuck is cast before Stalagmites?".
The answer is in HoN's engine because it runs at 20 frames/second or 0.05 seconds/frame.
Thus, if Stalagmites impacts the enemy in less than 0.05 seconds after Chuck is cast, then Chuck will include all 4 Stalagmite instances. Note that Stalagmites does have a tiny bit of travel time and 0.05 seconds is a very small timeframe, so Stalagmites will literally have to be casted right on top of Pebbles to ensure that it impacts faster than 0.05 seconds.
For curiosity's sake, Stalagmites' travel time must be less than 0.05 seconds & projectile speed is 1200 units/second. As we know, distance = velocity*time, so the minimum distance is 1200*0.05 = 60. Therefore, the center of the Stalagmites spell must be less than 60 units away from the center of Pebbles.
This can potentially be more easily demonstrated in a time line (assuming perfect scenario and that Stalagmites travel less than 60 units from the center of Pebbles):
Chuck at t=0
Stalagmites cast at t=0
Stalagmites impact at t=0.2
Instance #1 impacts at t=0.2
Instance #2 impacts at t=0.45
Instance #3 impacts at t=0.70
Instance #4 impacts at t=1.05
There is a 0.05s grace delay in between the Chuck cast and the Stalagmites cast. This grace delay is lessened the further away the Stalagmites has to travel, where 60 units is the max distance it can travel.
As always, code proof for your reference.
Chuck's projectile.entity:
<?xml version="1.0" encoding="UTF-8"?>
<projectile
name="Projectile_Rocky_Ability2"
gravity="4500"
lifetime="1000"
useexactlifetime="true"
>
<onimpact>
<spawnaffector name="Affector_Rocky_Ability2_Units" target="target_position" />
<spawnaffector name="Affector_Rocky_Ability2_Buildings" target="target_position" />
<playeffect effect="effects/impact.effect" source="target_position" target="" occlude="true" />
</onimpact>
<onimpactinvalid>
<spawnaffector name="Affector_Rocky_Ability2_Units" target="target_position" />
<spawnaffector name="Affector_Rocky_Ability2_Buildings" target="target_position" />
<playeffect effect="effects/impact.effect" source="target_position" target="" occlude="true" />
</onimpactinvalid>
</projectile>
Stalagmites' affector.entity:
<affector
name="Affector_Rocky_Ability1"
radius="200"
lifetime="1000"
impactdelay="200"
impactinterval="250"
maxintervals="4"
targetselection="all"
targetscheme="enemy_units"
effecttype="Magic"
destroytrees="true"
>
<oninterval>
<playeffect effect="effects/affector.effect" source="target_position" target="" occlude="true" />
</oninterval>
<onimpact>
<damage effecttype="Magic" amount="25,45,65,75" />
<applystate name="State_Stunned" duration="1250" />
<applystate name="State_Rocky_Ability1" duration="250" />
</onimpact>
</affector>
Stalagmites' projectile.entity:
<projectile
name="Projectile_Rocky_Ability1"
speed="1200"
gravity="0"
modelscale="1"
model="/shared/models/invis.mdf"
traileffect="effects/trail.effect"
effectscale=""
impacteffect=""
>
</projectile>