How is cauculated the Split damage from Alpha strike

Split damage from Alpha Strike is calculated at resolution, not when you play it.

At finalization you choose the friendly unit and the targets — you can pick at most as many enemy units as that unit's Might at that moment (Rule 355.14.c). When the spell actually resolves, you take the chosen unit's current Might (including any buffs granted in the meantime), then divide that total among the locked-in targets, choosing the split at that point (Rule 355.14.e). Each target must get at least 1, so if the Might dropped between finalization and resolution and there's now more targets than damage, you choose which targets cease being targets — but you can't leave fewer targets than you have damage to divide (Rule 355.14.h).

Bonus damage (e.g. from Annie, Fiery) adds to the total before you split it, not +1 per target (Rule 715.3).