You always want to chapitalize based off of the liquid volume. I'm not sure about the skin:liquid ratio for muscadine musts, but with red vinifera grapes it is usually between 70:30 and 65:35, meaning that if you have 10 gallons of must you would have 6.5-7 gallons of liquid in there. I'm sure your past experience with muscadine can give you an idea as to what the ratio is with that grape.
I'm not sure I fully understand the second part of your question... If you chapitalize before removing the skins, but based only on the volume of the liquid that you estimate is there, wouldn't that basically be the same as chapitalizing based on the volume of the liquid you have after removing the solids? In any event, since you're not going to ferment in contact with the solids then I would press off of them and then settle & rack the juice before taking a sugar reading anyway since the dissolved solids will affect the hydrometer. At that point any chapitalization would be based off of the actual volume of liquid that you have.
In ML: 2006 Brehm Vineyards Malbec
In Oak: 2005 Brehm Vineyards Petit Syrah
In the Fridge: VR Cardonnay, Special Changes
On Deck: Belgian Pale
Commercial Wine Production