Peter, remember that calories count. Fat is high calories. Better is to calculate your calories so that you are below maintenance on the low-carb days and above on the high carb days. Your fat and protein would remain relatively constant.
So I do drop below maintenance most days even though my goal is bulking? Is that to prevent fat gain or trigger some fat loss on the carb down days?
I'm counting both calories and macronutrient grams. I usually eat around 3000 calories on low-carb days, 3500 on high carb days. I say "around" because I'm using Fitday but I know it's a false accuracy - lots of best-guess conversions to cups and ounces from grams, best-guess equivalents for foods I've never seen outside of Japan, etc. But I hadn't been putting such a wide caloric divide between my high carb and low carb days. I guessed on the total calories needed - I hate trying to figure out my activities and their caloric costs, especially since they vary so wildly. So I'm paying attention to my measurements and my scale. Around 3000 calories a day is slightly more than I need right now, based on some very slow weight gain.
Ironmaiden - that sounds pretty useful but I also think I'm getting the basics (calories from fat, carbs, protein) broken down by FitDay so I'm not sure I'd use it. Wish they'd track Omega 3s vs. Omega 6s, but they're too busy feeding me "See Your Abs" ads.