I would think the best thing to do is follow your daughters lead. Keep doing what you have been doing, I think if she wanted you to have the title Grandma, she would tell you...
if you have met her children....what did she call you to them? Thats probably what she wants you to be.
I was very upfront with my sons birthfamily about what their titles could be. I had no problem with his extended birthfamily holding their titles (aunt and grandma) however, when talking to others, they do have the prefix of "birth". That's the way I did it within my own family (with my birthsister), and it worked quite nicely.
But thats me
I still say, follow your daughter.