There's no calculator because there is no definitive answer. Each of the refeed gurus suggest a different way to conduct refeeds so it's up to you to choose which one to follow or use a bit from all. The basic rules of a refeed are these:
1. Keep the fat intake low.
2. High carb intake including high GI carbs. Don't attempt only clean carbs.
3. Carb and calorie intake is relative to duration of refeed. The longer the refeed the lower you can go, where a short duration refeed would require much higher numbers.
4. Don't use ECA as it can blunt insulin response.
If you're doing a full day refeed then I'd get at least 15% above maintenance calories and all of them from carbs. Basically keep your protein intake the same as it is on diet days and make up the difference in calories in pure carbs.
If you want to do a half day refeed then I'd recommend you divide your maintenance calories in half to represent the half da time period, then up it 25-50% for the shorter duration.
One important thing to remember about refeeds is not to short change yourself by trying to keep it clean or not eat as many calories. If you're not prepared to go all the way with a refeed then don't bother doing them. To get the ultimate benefit and really get those leptin levels up you need to get those carbs in your system.