Thanks for posting your query at DoctorSpring.com
I have gone through your question in detail and I can understand what you are going through.
As you are not using any other antidepressants, its safe to use saint johns wort . I wold still suggest you to go for the tablet or the capsule as they are having a standardized dosage and while taking the powder or liquid, the dosage may vary. That way you can take 100mg per day to start with and if required can be increased to 300mg per day.
If you still planning to take in tincture format where in you dissolve the crushed herbal powder in the alcohol,it can be taken one dropper full ones a day initially and can be be increased to two dropperful two time s a day.
look for the symptoms of muscle twitching, diarrhoea, abdominal cramps, restlessness, confusion, muscle cramps and if you develop any of them, the dose will have to be reduced or it needs to be discontinued.
Hope this helps
In case if you have any further doubt, do let me know.
Dr. Srikanth Reddy