The precise amount would be hard to pin-down, as it would vary depending on individual metabolic rates. Yet, I would suggest a daily range of 600mg to 900mg of Bacopa Monnieri standardized to 40% Bacosides. This should give between 240mg to 360mg of actives.
The reason, as you know, is that Bacopa boosts thyroxine (T4) synthesis. But because T3 is the metabolically active thyroid component, T4 (about 90% of thyroid hormone) must be converted to T3 (about 10% of thyroid hormone, but up to 10 times more active than T4) to produce required effects. T4-to-T3 conversion is governed by both thyroid-stimulating hormone and growth hormone (GH). Higher GH levels produce low T4 levels, due to greater conversion of T4 to T3. But, thyroid dysfunction can occur if T4 levels drop too low. This is exactly where Bacopa shines, as it continually boosts T4 levels, ensuring stable conversion of T4 to T3. The precisely required amount of Bacopa for T4-to-T3 conversion would, therefore, depend on the amounts of growth hormone and thyroid stimulating hormone available. My conjecture is that a daily dose of about 600mg to 900mg of Bacopa at 40% Bacosides would elicit the required thyroid synthesis.