Several spells—or up-leveled versions of spells—have language such as this (charm person At Higher Levels from the Player's Handbook):
The creatures must be within 30 feet of each other when you target them.
I believe the above statement is equivalent to "The creatures must be within a 30-foot diameter (i.e. a 15-foot radius)."
When playing with such spells and using figurines and mats, I and others have sometimes started to measure distances between pairs of targets, rather than simply dropping a circle template of 30-foot diameter to see whether these can all fit inside.
Am I understanding correctly, and are the two phrases in my title exactly equivalent?
Note 1: I understand the difference between targeting an area of effect, like a circle or sphere, and targeting specific creatures within an area.
Note 2: Similar language appears in other editions, for example mass cure light wounds from Player's Handbook v3.5: "One creature/level, no two of which can be more than 30 ft. apart."