The soft cap can not be determined by a level in one skill. It is quite close to the system of the salary. If I remember correctly, a potential of starter hits his cap at maybe 15k (this I am not sure about, could be 10 or 20 but anyhow..), and the DMI does not matter whatsoever. You can find the soft cap vs salary tables here in the forums, they are mentioned every now and then, I don't have the time now to search.
Here's the table with the German descriptions, but the numbers will be the same for every language:
01 Stadionsprecher < 5.000
02 Bankwärmer < 5.000
03 Wasserträger < 9.500
04 Reserve 11.500 - 13.000
05 Leistungsträger 21.000 - 24.000
06 Star 33.000 - 40.000
07 All-Star 43.000 - 65.000
08 Überflieger 80.000 - 100.000
09 Super-Star 125.000 - 145.000
10 MVP 195.000 - 230.000
11 Rekordspieler 500.000 +
12 Wunderkind 1.000.000 +
We have both said a lot of things that you are going to regret.