I just finished searching about 850 1919 P dimes. At the 1/500 rate determined from the search of 2100 1919 dimes, I expected to find 1 or 2 (since 1.7 in not a possible outcome). I found one more.
The original discussion is here https://www.cointalk.com/threads/so-just-how-rare-is-the-1919-ddo-dime.400899/ Your cumulative stats are 5 out of 2800 searched = .18%, and again one die out of 521 dies used = .19%. You got even closer to the expected result from a random distribution of coins from 521 dies, where all dies produced the average die life's number of coins.
Cool. Nice find! I wasn't aware of the 1919 DDO I have a bunch of them that I have metal detected! I will definitely check when I get a chance.