Mages can normally only use one powerstone at a time.
Idea: if we borrow "Cumulative +400%" from Affliction...
80x5=400 so by spending 80 more FP and taking a -80 skill penalty. That COULD be bought off for 80 points as an Average technique (T39) rather than boosting your Powerstone skill to compensate (more expensive, 4/level = 320 character points)
For those who don't pay for the technique, another option could be to just pump in more energy, like +100% for +4, +200% for +5, etc. That'd be a LOT more energy though. Especially since Enchantment has much more limited energy contribution than other forms of Ceremonial Magic (skilled only).
I'm not sure if that would be % of base cost (20) or % of modified cost (20+80=100) though.
The idea is an Cumulative Enhanced PowerStone (CEPS) could be used with another CEPS to power a single spell: BOTH would need to have the +400% cumulative boost though, not just one of them.
I guess this could allow 3+ powerstones (there's no limit to how much Affliction: Cumulative layers) but to prevent abuse maybe there should be some kind of built-in penalty like -2 to skill per additional powerstone fueling the spell?
If +400% had a cap on how much something could accumulate then that should probably reduce it somehow. Like maybe if we applied "Limited Use" to it?