Probably can be refactored like this:

```ruby
def calculate_base_atk
  send("#{base_atk_progression}_base_atk")
end
```

Or if it's possible for `base_atk_progression` to be `nil`:

```ruby
def calculate_base_atk
  if base_atk_progression?
    send("#{base_atk_progression}_base_atk")
  else
    # some default
  end
end
```