Motivation (1-5) x availability of resources (1-5) - difficulty of a language (1-10) = Your score (needs a name, I like the David score
... but I'm open to ideas).A key factor is all the numbers you enter you subjectively feel. No one can tell you that your numbers are wrong, because your odds of success are due to how difficult you find the language to be, or how available good resources are that you like to use and can use for success.
Examples:
My first L2 was French: Motivation 3.5 (I was in school, feeling motivated but I wasn't the most serious) x 5 (very available resources) - 7 (I thought French was very difficult at the time)= 10.5
Later at age 19 after I had the opportunity to go to France to Study French in a summer intensive program, lived with a host family, made friends who spoke exclusively French, the formula would change for that period to 5 x 5- 5=20 ... odds of success rising
That is an example how the formula can change with time based on changing lifestyle, gaining/losing motivation, more/less available resources become available, changing perspectives, etc.
Spanish after having learned French: motivation 4 x resources 5 - difficulty 2 = 18
Russian in University: motivation 5 x resources 4 (I had a scholarship to study it at my University, so I had resources available) - 8 = 12
Japanese and Mandarin: motivation 4 x resources 5 - 10 = 10
Polish motivation 3 x resources 3.5 - difficulty 6= 4.5
Dutch: motivation 2 x resources 4 - 3 = 5
Bengali: Motivation 4 x resources 2 - 6 = 2
You get the point.
It is not a perfect predictor, but for me, I think it is pretty predictive of my likelihood to get to at least a functional B1-B2 level in a foreign language... the higher the score, the more likely I'll get there with time.