Thanks
So far I have been using this format:
field_{field_group_name}_{field_name}
Everything has been working fine as long as I use a unique group name and label name. Which you need to do anyways.
I think there should be an option to leave it blank and it will use this format.
I did have issues with older versions of ACF with the field id, but with 4.4.1 we don’t have any issues now.