I had to crank the Fusion Ball damage up to 4760 for a pair to destroy a Battleship.

4760 power means a minimum damage of 2380 per fusion ball. It takes 3000 to destroy the battleship in a single hit, and even if you fire both weapons at the same time, the two hits are counted separately. (I support adding an OXCE feature which can allow both hits to count together.)

Your 4760 fusion ball has a 26.0% chance of crash-landing a battleship, regardless of whether it is hit by one or more of them. Your previous value of 4755 had a 26.2% chance of crashing it in one hit. The minimum weapon power to ensure it destroys it is 6000.

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Additional info from my testing--assuming UFO has no shields, armor, or other hit point bonuses:

As weapon power rises above the minimum needed to destroy the ship (half of the damageMax), the chance of destroying it initially goes up, but then for a while it actually goes down and reaches a critical low point (0%) when weapon power equals damageMax. Going above that, the chance once again rises until it reaches 100% chance when the weapon power is 200% of the total ship effective hit points. The highest chance of success between 50% and 100% of damageMax is IIRC 12.5% chance of success at 66.66% of damageMax. Thus to have a high chance to destroy the craft, weapon power should ideally be 150%-200% of damageMax or greater.