I need to know how long to cook potatoes at 350*F
Whatever your book told you about cooking time, forget it. It will only work if you use the exact same size and shape potatoes as the book author, in an oven which has the exactly same deviation from the temp on the dial as the author's (I have to yet see an oven which doesn't deviate). 

If you have a baking thermometer, set the alarm to 96 celsius, stick it into a potato and bake until it peeps. If you don't, you will have to check for doneness as James McLeod suggested. Luckily, you won't have much trouble if you overbake them, they will still be good. They will start breaking down if you hold them way too long, maybe half an hour after they got ready. But this means that you have a half an hour window of error.