You asked some very intuitive questions about running gables. The answers depend upon a variety of conditions and circumstances that will vary some with each house you do.
The first consideration in all trim work, aluminum soffits included, is aesthetic balance. You don’t want a short sliver of soffit anywhere if you can help it. So you have to look at any run, calculate the number of pieces it will take, and determine what the size of the final cut-off is going to be.
This may mean you will not begin with a full piece, because to do so might give you a tiny little ending piece at the other end... OR it might cause you to finish the run with one of the “V” grooves too close to an edge that may not be square. This will show up like a sore thumb, and make your job look bad.
Secondly, getting to the gables, much will depend upon just how each end of the gable is built... how it ends. And, again aesthetic balance comes into play. You always have to look at the beginning and the end of any run, and do some division math. Try never to end up with a “V”” groove too near an end... or worse yet, half under the channel strip. This looks very amateurish.
If your gable is “boxed” on the lower end (as most are) you’ll probably want to begin at the bottom, and stack upward. However, if it is open at the bottom end, it is easier to run from the top down, and just trim the last piece along the edge.
One thing that will make your gable end job stand out as having been done by someone who knew their stuff is this..........
Up at the peak, where both sides meet, if possible (sometimes it’s too steep to do this, but not often), either begin there – or work out the calculations – so that you can fold a piece on one of the “V” grooves, right in the center of the peak. If you do not begin at the top, this takes some careful calculating, and honestly is not child’s play. But it can be done. This makes a beautiful peak, and eliminates the need to put a tacky cross piece of trim up there. It also keeps you from being tempted to fold a peak piece somewhere out in the flat portion. This looks like a mistake.
But, no matter where you start, how you run, make sure to try to never have two dissimilar sizes of cuts right next to each other. Can you imagine how the peak would look if you had a full piece on one side, and a 1-1/4” piece on the other? Low-rent as can be!
Fascia. Make certain your overlapping pieces work with your most prominent visual angle. In other words, say you are standing at your entryway, and look up at a run of fascia down the front of the house....... You do NOT want to look into the end of an overlap. You want the piece closest to you to overlap the next piece further away. And it continues that way till out of sight. Same thing with front to back on the sides of your house. If the side of your house is easily seen from the street, begin running your fascia from the back of the house so that all the overlaps are visually softened from the street view. There is no hard-and-fast rule on this and each house portion's visual angle has to be considered individually.
And the cardinal rule! Never ever run a cheap looking short piece to finish out a run. Think ahead, and waste a piece or two if you have to to keep all of the pieces reasonably long and close to equal lengths.
There are a few other tricks - like fake 45 degree corners on the channel - folded, not CUT fascia corners - and trimmed, end matching 45's on flat corner panels. But this will get you started in the right direction.
