[3.5]Greater Produce Flame (PEACH)

[Temotei]

Bolded for emphasis. That's where I'm getting 20d6 from. As I said, it looks like the bolded excerpt indicates that it deals +1 1d6 damage per caster level, but looking at it objectively, it's not 100% clear what the hell it's supposed to mean. Alternatively, and more to the point of what you seem to want, "[...] a 30ft cone of Flame dealing 1d6 fire damage per minute left in the spell's duration [...]" is perfectly clear. If there are eight minutes left in the duration it deals 8d6 damage, if there are three minutes left in the duration it deals 3d6 damage. It's the "+1 per caster level" part that is making the damage difficult to determine, and the phrase is unnecessary anyway.

I think what's meant is 1d6 per minute remaining (let's say 5 minutes for maximum, since a 5th-level druid can get this) + 1 per caster level.

So, five minutes is 5d6 and one point per caster level is another five. 5d6 + 5 fire damage.