High pass image filter & cut off value...HELP!

I have to ask someone for helping me in understanding/interpreting one part of some code.

Well, if I want to perform high pass filter for image processing, can I write something like this:

%Image conversion to grayscale

IM2=mat2gray(bw);

%High pass image filtering

mIM2=imfilter(IM2,fspecial('average',90),'replicate');

sIM2=mIM2-IM2-0.01;

If my understanding is correct, by using imfilter on the image low pass filter is applied. So, when from the original image this filtered image is subtracted (mIM2) then high pass filter is performed? Am I right?

Also, I want to know how to define cut off frequency for high pass filtering on this way if way is correct?

Thanks in advance,

Suzana