Your calculations are correct.
One thing you should know is that with Yatra GVs you can't cancel the booking even if it is mentioned that the hotel booking is cancellable. Yatra GVs are one time use only. So take care of this.
Also you can only use similar denomination Yatra GVs in a booking (which I know you are already doing but wanted to highlight).
Edit: One more thing, if the price of the hotel is 15k after applying some coupon, then you won't be able to use Yatra GVs. Yatra GVs can only be used when no coupons are applied. In that case, I would suggest you to check Agoda as from my experience the final prices (cost-reward) comes out to be the same.