How to scroll down less than all the way to bottom of page (Infinite scrolling, pinterest)

How do I scroll a little bit less than scrolling all the way to the bottom the whole time? Instead of scrolling all the way to the bottom, I want to scroll maybe 50% of it to grab more content. Since beautiful soup is not grabbing everything I need. Here's the code I'm using for scrolling down on pinterest.

while True:
  #Scroll down to bottom
  browser.execute_script("window.scrollTo(0, document.body.scrollHeight);")

  #Wait to load page
  time.sleep(scroll_pause)


  #Calculate new scroll height and compare with last scroll height
  new_height = browser.execute_script("return document.body.scrollHeight")
  if new_height == last_height:
    break
  last_height = new_height

I will appreciate any advice or help.