> Start with maybe 30 inches of fabric so I can roll it down a few times and account for hems. For the width of fabric though...
Well, a pack liner needs to match the internal dimensions of the pack. So, for a 24x11x8" pack, you need four side panels, 2 off 24x11 and 2 off 24x8. Then you need a base piece 11x8. Since packs are generally rectangular-ish rather than circular, you're better off using a rectangular base piece with rounded corners. Which means that no pie is needed...
Then you need to allow seam allowances.
Of course, you're better off making the main body with a single seam, from one piece that's 24x(11+11+8+8), i.e. you just need to add up the lengths of the perimeter 'faces' of the pack to get the 'width' of fabric.
As you suggest, the 24" depth is probably better increased by a few inches to allow for a roll-top closure. Since this roll-top will form the top face (nominally 11x8), you will need to extend by an absolute minimum of 4" (8"/2) to close the top on each side. Allowing a 1" roll and a three-fold closure, that will need at least another 1+3".
It's really a bit like deciding how much paper you'll need to wrap a parcel.
Or have I missed something more subtle in your question...?