So I went away for 8 days recently and had 0 access to the internet during that time. Before departing I put a deck up in Platinum League that I had high hopes for but was unable to see it win a single duel before that time. When I came back my deck had been reduced to 1HP, had 50+ wins and 20+ losses but because I was unable to log on at all to monitor it over those 8 days, there was no way to tell at what point that drastic HP reduction had caused me to obtain so many losses. Also, if my deck had risen to any of the top 10 places then I will likely never know.
So my suggestion is this. Create a function that is able to tell you how well your deck performs each day while it is in the top 500 for a particular League. You can make this as simple or as complex as you would like, but I would imagine that keeping it strictly to the amount of wins/losses total (or accumulated to make it easier for people who dislike math I suppose would be another option) after that day + what rank it peaked at during that time would be more than reasonable. HP remaining at the start of each day would probably be good to include as well
For example, in the case of my deck that I left up last week (purely hypothetical):
Day 1 (submitted): 8 wins, 1 loss, Highest Ranking: 200
Day 2: 16 wins, 3losses, Highest Ranking: 170
Day 3: 24 wins, 6 losses, Highest Ranking: 150
Day 4: 30 wins, 10 losses, Highest Ranking: 140
Day 5: 38 wins, 15 losses, Highest Ranking: 180
Day 6: 45 wins, 21 losses, Highest Ranking: 250
Day 7: 48 wins, 30 losses, Highest Ranking: 416
Day 8: 52 wins, 40 losses, Highest Ranking: 470 (Removed from Top 500)
Day 9: Not recorded
Where
-The number of wins is the total number of games won by your deck over the entire time that it has been in the Arena
-The number of losses is the total number of games lost by your deck over the entire time that it has been in the Arena
-The Highest Ranking is the highest rank that your deck achieved during the time that it remained at a particular Age (i.e. during the time that the deck was at an Age of 2 days before it aged to 3 days).
-All stats are recorded for each day every time that a deck ages a day. This recording stops for one day and begins for the next the instant that the Age counter is increased by 1. For newly submitted decks, the recording obviously starts from the instant that it is submitted.
Not really sure where this would be visible from in game, but I thought I'd just throw this idea out there anyways.