As carpenters always say - measure twice - cut once. I also believe that patterns run big. "Wearing ease" is what they call it and you can end up looking like you are wearing a sack. Adding length to a pattern is fairly easy - and it does sound like she is tall - though we don't know how tall gramma is. Best bet -besides getting a new size pattern and knowing she will grow into it eventually - is to make an inexpensive prototype and seeing how it works. 5/8" seams are forgiving and adjustable!