training increase = calorie increase?

over the last few months, i've been able to drop about 20 lbs. i've been eating around 1700-1800 on an average day, closer to 2000 on a heavy work out day. my general rule is if i have two workouts on the same day OR if my hrm tells me i've burned over 1000 for a given workout, then i eat around 2000 for that day. otherwise, 1700 or so. this has been working well so far.

here's the problem. lately, my workouts are getting longer. over the last week my hrm has said 1600 twice and i haven't even had my long bike or run yet. as my distance and frequency pick up, how should i adjust my calories?

one idea is to use 1700 as my base (since that's where i end up most days). on a rest day or an easy workout day where i've burned <1000, i would stick to 1700. on harder work out days, i would add 200 calories for every 500 the hrm says i've burned. so, if it says 1000, i would aim for around 2100; 1600, i would bump up my calories to 2300, etc. does this seem like it would make sense? should i really be trying to make up some of those calories burned? my goal is to lose weight, but i also don't want to bonk during training, lose muscle, etc. on the day i rode 25 miles, hrm said 1750, and i knew i had only had 800 that day...and i felt awful, chilled, etc. so, i think i need an increase of some sort. however, i don't want my 1000+ calorie rule to give me permission to pig out three times a week. i need a system that makes sense and will keep my weight loss on track.

using this calorie calculator, 2581 calories/day would maintain my weight at my current activity level (working out 6+ times a week). 2065 would be gradual weight loss; 1548 calories/day 'extreme' weight loss. if this is right, i should still lose weight even if i have a 2300 day? or at least not lose ground.

any advice? thoughts?