There is no one that is optimized at 90mg (1 grain). For most people, it takes about 3-5 grains to get the Free T3 and Free T4 to the appropriate range (we hypos want to be close to the top of those ranges; FT4 at the top 1/3 and FT3 at the very top).
I would not blame the Armour -- I would blame the dose. Hopefully your doctor is progressive enough -- many of them aren't willing to prescribe more than 1 to 2 grains per day (also don't take it all at once -- multi-dosing is key).