How to use a QUEST staircase in 2-AFC?
There is a lot going on in your question. QUEST, as well as many other adaptive procedures, is well suited for a task like estimating morph distance in a 2-AFC paradigm. There are, however, a couple of points in your question, that make me think QUEST is not a good paradigm.

The QUEST paradigm is designed around setting the signal level to find the the "sweet point" of the psychometric function (something like the signal level that maximizes the Fisher Information). I am not aware of any psychometric function that has a sweet point of 75% correct (most are 90+ percent correct).

The second issue, is you mention a fixed number of trials. Typically, QUEST is used with a stopping rule that specifies the expected variance of the estimate (i.e., you run enough trials until the CI on the estimated threshold is small enough).

The easiest way to run these types of experiments is to use software written by someone else (that you trust). Pelli was involved in the early development of both the QUEST procedure and the Psych Toolbox for MATLAB/Octave. The Python based PsychoPy also provides the QUEST procedure and is commonly used.