I have an old wooden gate (@ 60#) being repurposed but I need to build new columns for it. The columns will be stone faced, 12 x 12 x 40 inches finished. The soil is hard clay so I can use it as the form using leveled rebar as my pour guide. The column will be 2x4 pt frame held with simpson ties, covered with mortar board and conduit running through for lights on top. What's the rule for a footing for a gate like this? I was thinking 18 x 18 x 18 inches deep. Thanks for your time!