How do I calculate what a successful reflex DC save when trying to dodge against the flaming sphere?
The DC for a saving throw against a spell is 10 + the spell level + the caster's casting stat modifier + any other bonuses, such as the spell focus feat . This is stated in the class descriptions of each class that can cast spells. Flaming Sphere is no exception and I can't see why you'd think it would be.
For example, if the spell was cast by a Wizard with 18 Int, the DC would be 16 (10 base + 2 level + 4 int).
2\$\begingroup\$ +1 but I'd add: +any other special modifiers from feats, class abilities etc. Foe instance a creature from Plane of Fire might have a racial feature that says "+1 to DC of any spell with the [Fire] descriptor. Granted, these are more rare for evocation type spells than other schools but they do exist. \$\endgroup\$ Apr 25, 2015 at 13:56
1\$\begingroup\$ @Ben-Jamin I always consider "plus any other modifiers" to be a given but I'll edit that in. \$\endgroup\$ Apr 25, 2015 at 14:52
\$\begingroup\$ Reference here as well: d20srd.org/srd/magicOverview/spellDescriptions.htm#savingThrow. I did not edit in since you mentioned a reference not this. \$\endgroup\$ Apr 27, 2015 at 15:33