Chances for a 69 to upgrade to 70?

I've picked up a dozen or so lowball PR and MS70 coins just because I'm so curious as to what makes a coin "flawless" as opposed to only nearly so. I can't tell the difference. Here are some dimes that I can't tell apart:

 

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I'd almost guess the fastest is well circulated classics. Vf or F dimes for instance pretty much slap you in the face unless you see an issue you need to look at closer

 

There are no exact numbers-just anecdotes. If you have a few 69 graded coins where the jump in value to 70 is a decent amount over the grading fee (yet 70s aren't extremely rare) then it is worth a try to send them again as part of a larger batch. If you can buy a 70 for not much more than 69 plus grading costs, then there is no point of trying the grading game.

I was once successful for a modern world coin. I cracked two 69 graded examples and sent them in with some other coins. One came back a 70 and the other a 69 (not sure which one was which as they both looked the same). It was worth the expense in my case (increased the value and helped me finish that set all in 70-without having to pay the 70 premium).