I’m tossing another tip into my on going list of priest tips and tricks:
When reading guides on raid encounters and instances, there are often measures of distance mentioned. Usually, you can ballpark your distance to a target through the use of add-ons or from seeing which of your spells are in range. However, sometimes you might need an alternate means of measuring. Mass Dispel, if you click on it, will bring up a circle to target an area with. That circle has a 15 yard radius (which makes it 30 yards across). You can use that to ballpark other distance measures.