For example, Turn 1: Pokemon A used Future Sight! Pokemon B used Splash! Turn 2: Pokemon A used Calm Mind! Pokemon B used Splash! Turn 3: Pokemon A used Calm Mind! Pokemon B used Splash! Pokemon B took the Future Sight attack!
Would Calm Mind boost Future Sight's power?
The damage is based on the Special Attack and Special Defense at the time of striking, not at the time it was used on the original target.
So the boost of Calm Mind would be factored into the damage taken by the opponente.
The damage dealt is calculated based on the Special Defense of the hit Pokemon and no longer from the one that was originally targeted. The damage is based on the Special Attack and Special Defense at the time of striking, not at the time it was used on the original target. This means that if you use Future Sight on a Pokemon with 100 Special Defense, and then your opponent switches to a Pokemon with 300 Special Defense, the damage calculations will be done as though the target has 300 Special Defense.
So basically, the modified Special Defense stat will be used in the calculation, rather than the original one.
Bulbapedia Hope this helps.