Need Help with Forecasting Impressions and CPM in DV360 for Specific Websites

Hello everyone, I'm currently putting together a plan in DV360 and need to estimate impressions and CPM for display ads that we plan to run across 25-30 specific websites. However, I've run into a problem: when I try to use the Open Auction option, it doesn't allow me to specify individual websites. On the other hand, the deals option does provide me with the forecast I need. Ideally, I want to use the Open Auction for my campaign. Is it possible to utilize the forecast from the deals option to run ads through the Open Auction in DV360? Or is there a better method you would recommend? Any advice would be greatly appreciated! Thanks!