"What's the limit for estimated calorie deficit for shorter trips?"
I don't think there is an exact answer to this question. I have seen that my body has adapted and can more efficiently burn body fat than it could several years ago. How do I know? I am fairly exact on the calories I take for a given trip. I take 100 calories per mile and I weigh full out about 200 lb. I watch for signs that I am hitting the wall which used to happen on a regular basis, but hasn't happened in years. (Thanks in part to Greg's advice.). I tend to do very high mile days often 40+ Miles. My best estimate is that I will burn a full pound of body fat on a long day like this, so I will operate at a deficit of about 3000-4000 calories. Doing this a single day is one thing, repeating this over several days when your glycogen levels are spent is yet another. For multi-day trips there needs to be an additional focus on replenishing your glycogen at days end. I generally will do this with a 600 calorie shot of carbs. This combined with a heavy dose of protein will set me up for the next day.
Bottom line, everyone is different and the trips they do are different. Would I recommend someone get off the couch and replicate the 3500 deficit, no way. But I firmly believe it is a very sound endurance fueling strategy that is scalable regardless of the mileage, terrain etc.