glass cannon viktor calculations

please correct me if i'm wrong should hit approximately 1009 ap at level 18 if i did the math right Q 120+.20 ap + 90+.5 ap [at level 18 (5x18)] W 0 E 250+.70 ap + 40% R 350+ 7(90+.20 ap)

so assuming they get hit by both parts of E (or just the explosion) and stay in your ult for 7 seconds you do 5170 damage (before damage mitigation) with 1000 ap not including lich bane procs (should be able to add roughly (.75 level 18 ad + 500 ap) to 4 times in the 8 seconds you'd need to apply the damage of the ult) before factoring in the 15% of max hp damage from dfg or the 4 seconds of boosted damage from dfg is this an optimal glass cannon build, or would void staff be better than rylais for true glass cannon? what would be a reasonable amount of time to assume they stay in your ult? were my calculations accurate? should this be under tips and tricks? am i asking too many questions?