I had constant temp at 68. I can gauge pretty well how much was lost by the increase in my blow off bucket, which has graduated gallons marks. Both that and the level visible in the glass carboy look like I've lost about a gallon. I had a blow off with my last all grain batch also, which wasn't nearly as high OG (1.065) but I didn't lose volume like this.
wyazz, thanks for the tip. I'll check into fermcap.