Folk magic is described as petty magic available usually to most folk. This has led to design decisions that folk magic spells in RAW are ‘fixed’ in strength and placed low in the ladder of magical strength. It can also be stronger like it was in history of the game. If you want variable folk magic spells from AiG you could use these rules. To keep the relative strength of the magic systems aligned the spells variability is controlled by the Folk Magic skill level required and INT required.
Bear in mind that this makes the folk magic spells significantly more powerful (and also weaker in the lower end of intensity) and take this into account on your campaign’s magic levels.
You might also want to constrain the intensity attainable per spell to certain cults and/or cult levels. So for example Healing intensity 1-3 are available to everybody and Healing 4 to shamans of any cult. Healing intensity 5 or higher is only available to Healing cult or Healing Cult’s Priest level. If you use progressive version of the spell it replaces in its technical effect the RAW version.
Caster does not have to use full intensity available.
The spell table shows how each of the spells are modified by intensity. Those spells not on the list stay fixed as they are.
With this system for example:
Bladesharp adds dice bonus to blade weapon damage. Only dice bonus is magical. For example intensity 4 would give +1d4 added damage and cost 4 MP. It would require 61% in Folk Magic skill and at least 10 INT
These rules distilled from AiG are used with permission and are slight modification in presentation from the original.
The spell table and skill/int table is in here
Other useful charts would be in here