Perhaps they got it from the fact that in RMFRP, if you declare a full melee attack, you get a +10 to OB (RMFRP, page 217).
Anyways, in RMFRP, an attack takes 60% - 100%, and the standard rule for RM is that if you are performing an action that has a percentage range, each % under the maximum range equals a -1 to the skill.
There is no official rule that allows ful OB when using 90% activity.