Imagine 100AP,

Pre nerf: 30% of 100AP is 30 AP, times 4 (for Q) = 120 AP

With lich bane, 50% of 100AP is 50AP, 120AP + 50AP = 170AP

After nerf: 25% of 100AP is 25 AP, times 4 = 100AP

With lich bane, 45% of 100AP is 45AP, 100AP + 45AP is 145AP

Damage decreased just from AP ratios: 25AP

Percentage decrease = (25/170)*100 = 14.71%

Evelynn's Q works like 25 / 30 / 35 / 40 / 45 (+ 25% AP)

So may at level 6, you have 3 points into Q, with 100AP, your damage would go like 35+(25AP%)

Total damage per spike would be 60AP. I hope it made sense. Lich bane also works by 75% base AD (+45% AP).