There are two ways to create custom ringtones.
1)Through an android app from the play store
2)Real player mp3 cutter which cuts the mp3 from the desired starting time (from where to start) and the end time as well(till where it should play)
Now for this-
"considering that I don't want to cut the track to the point that I want my ringtone to start from?"
Is completely irrelevant because without specifying/cutting the start time how will the audio file start to play as all the audio/video files start from 00:00.
The audio file cannot randomly start anywhere you like . It has to have a point where you need the song to start(00:00 till the end but not from 2:50 till end).
Ex:-Suppose you have Beethoven audio file in which you want the phone to ring from 4:00 till 4:35 i.e. 35 seconds then with the help of a mp3 cutter you will set the starting point from 4:00 and ending point to 4:35 .After you have set the slider then you need to cut that particular portion and it will be set as 00:00 to 00:35.This song will not replace the original Beethoven song but will add an extra song of just 35 secs as a ringtone.