If one was willing to rewrite the DMG rules on centering spell effects, and ignore the contradictory spell area diagrams, then I'd suggest replacing them with the following simpler rules. First, keep the "majority of a grid square" guideline, and use a little integral calculus, so that circular areas are as exact as possible. Second, have all spells centered on a square instead of a line-intersection, which is more intuitive to players, and manages to avoid the special-case situations for meteor swarm and effects emanating from a creature (as seen on the preceding page). The Radial spells actually end up having much easier-to-remember areas of effect (for example, areas of 5 ft., 10 ft., and 15 ft. radius are simply 1x1, 3x3, and 5x5 squares under this technique). They can also be used to evaluate the threat areas of creatures with long "reach", by just adding 5 ft. to the actual reach and looking at the Radial diagram of that radius. This ruling also has the side effect of making all of the Arc-type effects, aimed at one of the eight spaces surrounding the caster, symmetric and more closely matching the number of spaces covered in their straight-line and diagonal forms.

The main drawback the this alternative system is that it always generates smaller-than-actual radial areas, particularly at the lowest distances, for which players may reasonably complain; however, it has several advantages in its simplicity. However, since the DMG "guidelines" for adjudicating spells on a grid do not appear in the System Reference Document, it can be argued that they are not part of the core rules, and hence this alternative is a valid method of determining them. All that would be required for this revision is to supply players with spell templates so they can easily see what spaces an effect covers, as I've done below.

Spell Templates: Radial Areas (bursts, emanations, spreads, cylinders)

Spell Templates: Arced Areas (semicircles, quarter-circles, cones)

To return to the main page for spell areas of effect templates (featuring expanded discussions and definitions), click here.