Couldn´t stop thinking of it:

NT @ 54% Haste, 24% Crit, 20k SP (18 ticks) - no lvl 90 buff

9.939 DPS and 116174 DMG

Duration: 11,69s

NT @ 54,5% Haste, 24% Crit, 20k SP (19 ticks) - no lvl 90 buff

9.972 DPS and 122629 DMG

Duration: 12,30s

Summary:

We have 6.455 more damage with the 19th tick NT and a longer duration. In a 450s fight it would be like:

54% Haste (18 ticks): 450s / (11.69 + 1 GCD) = 35,46 NT

35.46 x 116.174 DMG = 4.119.530,04 overall NT damage in 450s

54,5% haste (19 ticks): 450s / (12.30 + 1 GCD) = 33.84 NT

33.84 x 122.629 DMG = 4.149.765,36 overall NT damage in 450s

With 54,5% haste, we do 30.234,96 dmg more and saved 1.62 GCDs for an another ~50k Frostbolt (20k SP, 24% crit) with 1.3s cast time

Thats (together with extra frostbolt) 177,77 more DPS. Nathyiel is absolutley right, a breakpoint makes no real difference (if my math is correct, of course).

@ stX3

Got my spreadsheet from the fire mage guide, but yours is better. I was looking for that, thank you.