When to begin letrozole


I had a miscarriage at 9 weeks and a D&C on 4/11. My OB said the best rates of a successful pregnancy are 0-3 months after a D&C and she prescribed letrozole (how we conceived the first time). I am still having a very very faint line on first response tests (negative on the really inexpensive pregmate tests). I was instructed to begin the letrozole as soon as I got a negative. My levels were 29 on 4/26. Is it normally for the HCG to drop and then linger steadily? Should I go ahead and start the medicine or wait?