My suggestion is not to worry about the exact PPG. Just use 32 ppg as a ballpark, figure something like 70% efficiency, and formulate the recipe around that. Then take a gravity reading after you do the mash (make sure to cool the sample to whatever temp your hydrometer is calibrated for!) to see how much to adjust your extract amounts. If you're aiming for about 5%, you want an OG around 1.053 or so (which includes the contributions from whatever extract you plan to add). So depending on how efficient your mash turns out to actually be, you may have to add more extract or less to hit that mark. That's how I do it, anyway, and it's been working out juuuust fine!