IIWY, I'd not bitter-hop it so much, and use those valuable hops elsewhere. They say that humans cannot perceive any difference in bitterness past 100 IBU's---and I tend to agree. I tried ohiobrewtus' Hopacles next to my Destroyer of Worlds---his had like 250 IBU's, mine like 135---and there was very little perceivable difference in bitterness. When hops were $10/lb, sure, why not? But now that they're going for $5/oz or more, depending on the variety, it makes little sense to me to "waste" them by going above like 120 or 130 IBU's.
My $0.02? Take half of the 90-min hops and mash hop with them. Then take the 45 minute additions and divvy them up between 20 and 1 minute. You could even continuously add every minute between 20 and flameout. So, just use that 90 min addition to get your IBU's at around 130, and work from there.