There was an incident today that led SheShe and I to redefine foci and their use. Freya managed to boost a spell to tier 10 (previous allowance) and did 89 damage in a single turn, with a single spell, as a result with a focus that only cost

A spell may be boosted (upcasted):
Previously:
1. It will cost the next level slot cost, so a level 1 spell boosted to level 2 will cost 2, the same spell boosted to level 9 will cost 9. You must be capable of casting level 9 spells without a foci.

2. If you happened to have a 9MP foci, it can be used to boost any spell to level 9 even if you cannot cast a level 9 spell, but it must be a spell you can already cast and beyond one level of boost, the results are unpredictable.

Currently:
1. It will cost the next level slot cost, so a level 1 spell boosted to level 2 will cost 2, the same spell boosted to level 9 will cost 9. You must be capable of casting level 9 spells without a foci.

2. If you boost using a focus beyond your ability to cast that spell normally, it will cost the spell slot for each additional boost.

Say you can cast tier 2 spells, and you would use the focus to boost the spell, you could use 2MP from your pool to cast the spell at level 2 and boost it using the focus as follows:
Tier 3 = 2+3 = 5MP
Tier 4 = 2+3+4 = 9MP
Tier 5 = 2+3+4+5 = 14 MP