slope

I've noticed this a few times recently that the course rating and slope on the scorecard differs from what GHIN has in their system. It's typically not enough to matter, aside from the occasional rounding issue when calculating course handicap, but am still curious what the discrepancy may be...

Is there a quick way of adjusting your club distance for slope? For example if I was 100 yards away from the pin, and the flag was 20' below me. How much would I adjust for the slope? Hoping there is a quick formula or rule to calculate this. Thanks, SHIFTY