Welcome to the board. From the picture you posted, I'm not seeing anything to evaluate for the chinstrap but for sure you have a great goatee going. The chinstrap doesn't need much hair under the chin so perhaps you have enough hair for that already... show us some side shots after a week or two of growing. There is really nothing you can do to get your beard to develop faster so my best recommendation is to grow what you can and be happy with it. Good luck.

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