Generally I've preferred using the showProgress
function in ImageJ, since it doesn't require a new window:
showProgress(progress)
Updates the ImageJ progress bar, where 0.0progress<=1.0. The progress bar is not displayed if the time between the first and second calls to this function is less than 30 milliseconds. It is erased when the macro terminates or progress is >=1.0. Use negative values to show subordinate progress bars as moving dots (example).
showProgress(currentIndex, finalIndex)
Updates the progress bar, where the length of the bar is set to currentIndex/finalIndex of the maximum bar length. The bar is erased if currentIndex>finalIndex or finalIndex==0.