The spells rank counts as one lower than normal. Rank, not Power Level.
So if you have the spell Mend Wound (Rank 2, Healing, Druid) written down on a scroll or in a grimoire, you can cast this spell with the help of this written aid. The spell take some extra time casting but also counts as a Rank 1 spell. If you are a druid with Rank 2 in the Healing talent and cast this spell with the help of the written aid, the spell is cast as a spell one Rank below your Rank. This mean that you can make use of the Safe Casting rule. In this example this mean that you can opt to roll one less Base Dice when you cast the spell. Safe casting doesn't lower the Power Level of the spell, it just mean that you can cast this spell safer (less chance of a mishap and yes less chance for overcharging).
“Thanks for noticin' me.” - Eeyore