Optimization problem for threshold calculatio​n/identifi​cation

Dear MATLAB enthusiasts,

I want to calculate a threshold for vector A so that B is maximal. In this case B is a bivariate variable with "+1" for "ok" and "-1" for "critical". I want to find a threshold for A so that B is "ok" 100% -> in the case below that would be 0.4. Finally I need B to be "ok" in 70% of the cases --> in the case below that would be 0.35.

Is there a MATLAB function which can identify the threshold parameter for A so that in B the sum is max?

Plus I need a Threshold so that only 30% of the values are critical like 0.35 in this case – can that be done as well?

I would appreciate your help very much.

Thank you and best regards William

PS: I don't think that sorting is a solution I'm locking for due to my second question and the involvement of two vectors.