I saw a "formula" on a Web site that was pretty helpful for adjusting patterns to a new stitch size.
Let's say your pattern calls for 30 st over 4".
Knit a swatch, and you have 19 st over 4", not 30.
So divide 19/30 = 0.63. That is your "multiplier." In other words, you need 63% of the number of stitches called for in the pattern to use the gauge in your swatch. 63% of 30 = 19. So if you are supposed to have 114 st/row as the pattern calls for, multiply that by .63 to get the adjusted number of stitches to use the gauge of your swatch (114 x .63 = about 72 stitches). So instead of a row that has 114 stitches, you have a row of 72 stitches to get the same size.
It works the same way for row gauge.
It gets a little trickier for armholes and necklines. I haven't tried that yet, but I imagine it's not that hard to figure out (famous last words....)
I saw a freeware program called Knitting Companion (I think that's the name) It's a simple thing, really geared towards machine knitting, but it works for hand knitting too. It tells you the adjustments in stitch and row numbers you need to make.