If there is any regularity in the height description of ornamental grasses (and I haven't seen any), it would be that the stated height is to the plumes.

I'd suggest writing down the name of the grass you see, and research it's mature height online, at sites like bluestem.ca and this forum. Bluestem lists both foliage and plume height. Keep in mind, however, that mature height can vary from zone to zone, and garden to garden. There are many factors which determine mature height; soil, water and sun for example. It's impossible to accurately predict your results.