Re: slpices in panels
In the area where you are splicing, imagine using a saw and cutting a slice all the way across the entire panelboard, complete with overcurrent devices, trim, etc. The space where you are splicing, looking in at the end, is the area that you have to calculate for fill. This applies to either side or the top or bottom depending on where you are splicing.
Unless you are bringing all the wires to the end of the panelboard and doubling them back a couple of times, you will not generally have a problem with room.