There are no good quick answers without knowing what shapes/sizes you want. For your example of half and little more might not give you enough yardage for wide sashing strips, you might be 2" (or whatever) short of what you need to cut even if the overall area of yardage is enough.. For a rough estimate, I would multiply the stated yardage by .75 -- so 2 (yards) x .75 = 1.5 yards. That might leave you with some scrap -- I'm fine with that
When I figure out yardage, I use a WoF (width of fabric) as 40". Sure, sometimes you can get in a whole 2" or slightly larger piece over that, but you'd be surprised at how many fabrics are just 40" usable.
Remember that for HST, while normal seam allowance is .25, the ends of triangles is 7/8 -- I use a full inch for figuring yardage. And cutting and sewing too -- then I trim down to the proper size.
As GingerK said, count the number of pieces of each color you will need for a block. You can either write down the measurements or use graph paper --- but you should write down each step and remember your seam allowances!
Calculators either in your hands or on your computer are a great thing! As is basic lined or graph paper. I also keep tape in my sewing kit so I can put together pages if needed (tape on back, it typically doesn't allow writing over). For a twelve inch (finished) block you will definitely need more area than a standard piece of 8.5x11 printer paper.