There were questions about how many stitches to cast on, when working an A-line. That can be kind of subjective. I've seen A-lines with bottom hems that are positively voluminous (1000% or more). That's not going to be any fun at all to knit in plain stockinette, and depending on the weight of the fabric, it wouldn't look good.
So all I've got are guidelines.
250% is a pretty good rule of thumb. If you're using a heavier fabric (3 stitches per inch or heavier), I'd consider 200%. For thinner fabrics you could go up as high as 500% but I'd think twice because DAMN, that would be a lot of stitches.
250% with a hem is what I'd do. You need to decrease down to 100% by the time you get to the arm pits, or 100% + 8 stitches if you're doing a cardigan with a steek. To figure decreasing, first you'd need a row gauge and a measurement for how long you wanted the 'skirt' of the jacket to be.
Let's say your 100% figure is 100, for the sake of argument. And you need 75 rows to make your skirt long enough. The cast on would be 250 stitches - 250%. You would need to decrease 150 stitches to get down to your 100% figure (total cast on, minus your one hundred percent figure, equals how many stitches you need to decrease). To figure out how many stitches to decrease per row, you divide 150 by 75: you get two stitches per row (total number of stitches to decrease, divided by the number of rows you need to do it in, equals how many stitches to decrease per row). To put it in knitter-ese, then, cast on 250 stitches, and decrease two stitches per row until you arrive at 100 stitches and the arm pits of your sweater.
You can either distribute the decreases randomly over the skirt to make it a circular piece, or you can decrease at two distinct points to make it more angular. Up to you.
There you go. No swooning.