It really depends on what you're making. If you're making a scarf, for example, you can change the number of stitches you cast on. If you're making a sweater, that would be difficult to do and still be sure you end up with a sweater that fits.
In order to get your gauge to 8st per inch, you'll have to go down in needle size. Thinner needles would give you more st per inch. It's hard to say if you'll need to go one or even two sizes smaller.
Another option, which I've done, but can't give a formula on, is if its a garment, is to make a smaller size than you normally would, but this is risky.
Your stitch gauge is the most important to get here. The number of rounds can be adjusted as you go, but you can't change the width.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry