All non-special powers have a damage number. At 5/5 upgrade level that number is 2x the un-upgraded level. Overpowering an ability 2 stages doubles the base damage of that attack (ignores upgrade level!) and increases the success chance and duration of applied effects.

All damage dealt is subject to a variation of 50% to 150% (minus 1) of the base damage. This includes thrown objects, special types like Rally the Troops, and the duration of effects like blank and stun, but not the success chance of those effects.

Additionally, for melee attacks your strength level is considered - 4 strength is the zero-point and each point of strength above/below that affects damage by 20%. This amount affects the base damage and is considered when overpowering. For instance, Minuteman's strike at 1/5 upgrade does 14 damage (Medium = 10 + 40% bonus for having 6 strength) by default. Overpowering ramps up to 28 damage. At 5/5, Strike does 28 damage (adds 100% to the base 14 damage = 28) but overpowering does 42 damage (28 + 100% of base damage = 42).

Rally the Troops and 300 are the same damage type. They both multiply the damage, stun duration, effect duration and knockback of the next instance of damage you deal. The only difference is that due to being a special, 300 percenter is unaffected by overpowering and damage fluctuation.

After testing, I have concluded the formula for damage multiplication is:
mult = (rally damage / 25), with a minimum of 1.

300-Percenter therefore applies a base 75 Rally damage to the user. It's coded to always be a 3x multiplier with no fluctuation.

Note the multiplier is applied after other damage calculations. Hitting a target with Empathy active won't result in a boosted damage return to the attacker, and a weak but boosted attack won't guarantee damage against a Highly Resistant target. (for instance, minute missile occasionally rolls '0' damage against a streetlight)

Minuteman's Rally base damage clocks in at 15 and peaks at 30 with 5/5 points. With overpowering that's 45 points applied to closeby allies, so a nominal multiplier of 1.8x that can swing from a low of 1x (can't go below 1) to a peak of 2.7x

So when Minuteman uses 300-Percenter and follows up with overpowered Rally the Troops, he will boost allied damage from 2.7x to 8.1x with a most likely of 5.4x damage, provided you're close enough to avoid area damage falloff. That's how you slap good in the campaign.

Consider storm bolt which comes with man o' war at 1/5 = 10 damage. When overpowered, the damage spread is 10-20-29 damage. With Minuteman's boost that's 30-108-234 before considering Mr. Mechanical's 4x vulnerability to electric attacks. Those are some good odds of one-shotting Mr Mechanical without any investment beyond recruiting the hero.

This kind of boost applies to most power types. Although it won't increase the odds of applying a state (only base damage, overpowering and resistances influence that), it will absolutely boost the effect duration, stun duration and knockback strength. Irradiate and Acid Burn are capped at 5 damage by an internal variable so they don't appear to benefit at all.

You cannot snowball the effect alone. This is because only one instance of the effect can be on your character at once, and the initial rally is 'consumed' immediately after the updated rally is applied, so you get nothing.

If you use a second hero with rally however, you absolutely can stack the multiplier exponentially. That is how you slap REAL good. In FFvTTR minuteman and The Bard can accomplish this together.