Setting a before_action based on a conditional statement within a method

I have a controller that sorts the order of objects based on their numeric position:

<%= form_for @subject do |f| %>
  <p><%= f.label :position %></p>
  <p><%= f.select :position, 1..@subject_count %></p>
<% end %>

Presumably if someone wanted to create a new subject, an additional numeric position would be added to the select box. So for instance, if there were 5 subjects and someone wanted to add another there would be 6 positions to select from.

Here's how I currently have my controller set up.

def new
  @subject = Subject.new
  @subject_count = Subject.count + 1
end

def create
  @subject = Subject.new(subject_params)
  @subject_count = Subject.count + 1
end

def edit
  @subject_count = Subject.count
end

def update
  @subject_count = Subject.count
  ...
end

private
  def subject_params
    params.require(:subject).permit(:name, :position, :visible)
  end

  def set_subject
    @subject = Subject.find(params[:id])
    @subject_count = Subject.count
  end

I'd like to keep this code DRY by using a before_action that would add 1 to the Subject.count only on the new and create actions in one method. Is there a way to setup a conditional that I could accomplish this? Here's what I had in mind:

  def_set_subject_count
    # if new || create actions are requested
    #   @subject_count = Subject.count + 1
    # elseif edit || update actions are requested
    #   @subject_count = Subject.count
    # end
  end