Where: t = tape out date week, r = the number of metal layer required, and b = the number of base layer respins required.
So, using our equation, assuming 2 metal layer revisions before launch (same as Cypress) and knowing that April 21st would be Week 16 we arrive at the following:
Since this will be essentially the third generation of GPU's produced on the 40nm process at TSMC, perhaps we can assume that ATI has a good grip on the process by now and will be able to produce S.I. with only 1 metal layer revision before production. This would yield: