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You should specify the noverlap on the segments of your time data, not on the value of zero-padding you may or may not use for the DFT.
So base it on your WINDOW argument
WINDOW = 300; NOVERLAP = round(0.75*WINDOW);
Or if WINDOW is a vector,
NOVERLAP = round(0.75*length(WINDOW));
Also, you did not give the error you obtained, but I'm assuming that error is because 0.75 times an arbitrary integer is not necessarily an integer.
Still, what I said initially is what you should do. Base the number of overlapped samples on your window (segment) length, not on the value of your zero-padding. In many applications you may choose NOT to zero-pad the windowed Fourier transforms at all.