How much damage does the Primary Attacker Rune give?

The 70% lands on one player only, so whoever touches the rune decides where the buff goes. It does not spread across the party. That makes the pickup a team decision rather than a race: if the wrong person grabs it, the bonus goes on someone who is not built to use it.

This is why the Primary Attacker offering suits coordinated teams built around one exceptionally powerful main carry. Agree beforehand who your main DPS is and let that player interact with the rune. A group of evenly matched players without a clear carry gets less from it, because 70% on one person is all it gives.

For a sense of scale, Nimbi Burst raises the whole team's damage by 15% for 20 seconds, or 30% with Turbo's four passive stacks. The rune is the opposite trade: a much bigger number, concentrated on a single attacker.

Not settled: How the rune's 70% combines with other damage bonuses, such as Nimbi Burst or Pawnie's Pursuit trait, is not settled.