There is so much to lagging body parts. I don't think adding a few extra cals that day would make too much difference without changes in other areas. Adding effective intra nutrition and using aids to increase the uptake of those nutrients whilst training lagging body parts is something that could be very beneficial. Although if you're not training that lagging body part effectively it doesn't really matter how many cals you add. So the first area I would look at is establishing a better mind muscle connection with the lagging body part and try to improve training that way. Then I would look at adding intra nutrition on top of things. Later/now you could also look into things such as pre workout orals/injs, pre/post insulin and/or igf-1 lr3 along with plenty of aminos and carbs. But sure with better training and increased overall cals that day you could improve your lagging areas. What is it you want to improve?