If you have the time and the money, I suggest going to the ANA's grading course at summer seminar. You and all the other YNs can learn the ropes on grading and hopefully avoid another sad experience like this. You could also just take 4 points off what you think the coin would grade to get an accurate grade.
Most coins I look at graded have 0 marks at 65, 1 or 2 barely noticeable marks at the most, after that it is all eye appeal and market grading. Grading until uncirculated is textbook, after that, experience is the only way to learn.
The majority of dealers I communicate with regard PCGS with a higher esteem than NGC, and bluesheet prices mirror this. Maybe that "TPG" you cracked the coin out of was probably just an SGS or PCI holder.