For me it's around 2:1. After 2 lv30, I usually get the 40. Stage 37. Maybe you have to stay in game for increased chance? When I open the app I first duel lv 30 Tea, then auto duel a few duel island matches, then lv30 Tea again, finish remaining duel island and finally lv 40 will show up.
I am topped out on stages, most characters are level 45. That seems to have nothing to do with getting a lv 30 or 40 like some suggest. I have had 3 lv 40 and 15-16 lv 30. I came here because I just got my 10th lv 30 IN A ROW. First 2 were 40s. Then I had 2 or 3 30s. Then 1 40. Now straight 30s. I lost to the second 40, and I lost to one of the 30s by mistake.
After that 1 lv 40 (for a grand total of 3 Lv 40), I have had 9 straight lv 30 (for a grand total of 27). Ratio is 1:9 for me right now...sickening. Got the purple light though...waiting on just one voltanis.
I only got 5 LV 40 (got two more) and about 35 LV 30. I only got 1 needed card (the purple light) from the LV 40s, I got the others from the event rewards and the duel rewards of the LV 30s. Took about 40 duels. I stopped Duelist Island with 2,500,000 points...and got those rewards too, but I got all of those cards at 1,500,000, the next million was just enjoying the event. Done and done.