Most of my patterns are older or foreign and getting the yarn used in them is impossible so I usually just use something close in weight that I like that will get me gauge. But overall, I tend to knit things where gauge isnt as important, such as toys or scarfs. The few items of fitted clothing I have knit have used math formulas and actual body measurements for stitch counts rather than standard sizing for stitch numbers which has been nice because I am not a standard clothing size and would need to do alterations to most patterns.
I let my computer do all my calculations using a spreadsheet. Much easier and less error prone.