Basal Energy Expenditure (BEE) plus an activity factor:
the amount of calories one needs for maintenance of current 
weight with out considering any acticivity factor (confined to bed). 
The BEE plus adding calories for an activity factor and calories 
for consistent weight gain gives you your weight gain caloric needs.

Rember everyone is different and even scientific equations can be off slighty! Therefore, adjust your calories accordingly and keep track of your progress by scales and body-fat analysis.