As of v3.7, in5 automatically vertically centers flipbooks within the window.
To vertically center your flipbook, simply update to the latest version.
If you'd like to turn off vertical centering, you can apply this bit of JavaScript as a Resource:
vertCenterFlipbooks = false;
To vertically center a Slider see:
https://help.in5.us/en/support/solutions/articles/26000037902