I have never performed a 7 minute interval but, using WKO+...ahem, I plotted my 7 minute power and my 18 minute power. For me my max 7 minute was 356 watts. My max 18 minute effort was 331 and my FTP is around 314. So in my case my 18 minute CP is 107.5% of my CP7 power. If you have similar ratios your 18 minute power would be (310/107.5%) 288.37 watts. I'd give myself another 5 watts for adrenaline since it is a race.