Formula Required - Calculated Field - Group by Range of Name first Letter (eg. A-K or L-Z)

I am looking to split a territory by Name of the organisation, EG Companies beginning with 0-9 or A-K and Companies Beginning with L-Z by populating a custom field called territory split with a value (say "A-K" or "L-Z")  and thought there might be a formula that would work as a calculated field.

Thanks in advance for your assistance.