KNITTING - gauge swatch for circular knitting?

I'm planning on knitting a (stockinette stitch) top without a pattern. So my gauge swatch will help me determine how many stitches I need for the person's body measurements. However, I'm gonna knit the top by circular knitting and I'm confused on the logistics of a gauge swatch for circular knitting.

If I knit the gauge swatch in stockinette using straight needles, will it give me the correct math?

I'm hesistant to try gauging with circular knitting because the length of the circular needles will make such a big swatch that I'll waste alot of yarn and I don't like to part with my yarn before a project for fear of running out of yarn mid-project.

No, a gauge swatch knit on straight needles, knit flat will NOT be accurate for knitting in the round (sorry, kids). The stitches in circular knitting are formed in one direction all the way around every row and this affects how the gauge comes out.

You have two choices here, if your sleeves are knit in the round, start to knit a sleeve first and measure gauge above the ribbing/hem. Not only do you have a gauge swatch, but if you have to try a couple of different needle sizes, it will generally block out and be unnoticeable and you'll have a sleeve, or part of one, finished. Second, take some double points or two circs, cast on about 30 stitches and knit a circular gauge swatch that way.

Yes, it will give you the same gauge. Your gauge won't change if you are using the same size needles, regardless if you are knitting something flat or using circular needles.

Yes, you can knit your swatch on straight needles, as long as they are the same size as your circular ones.

I think if you use the same size straight needles that your circular needles are to knit your swatch that the gauge will be fine.