1) This forging calculator is based on eAthena's (revision 13377) potion creation code, it could give different results from official servers.
2) It is not documented anywhere officially that Vanilmirth's Change Instruction Skill will increase chances of brewing, however it is found in the eAthena code as of r13377. If you don't believe it will help, you may simply change the level to below 1 to turn it off.