If we are talking about mild steel here you are not likely to get it too hot so just heat those pieces as hot as is practical especially right at the weld. While they are still hot use the large tip you have and see if you can melt a puddle in the 1/2" plate and move it along the intended weld area. Remember you are trying to fuse the two pieces to each other not trying to stick them together with the rod so melt the base of both pieces and add filler rod as needed, don't try to melt the rod onto either part. Try to maintain an equal area of the puddle on each part and feed the rod in from the back (my method anyway) just adding metal from the rod to the molten puddle, never melt the rod and try to fuse it to the parts. Give it a try and let us know what happens, it may seem a bit difficult at first but trust me you will get the hang of it then it becomes much easier. You are doing yourself a big favor by learning to do this and if every welder would take the time to learn to gas weld first they would all be better welders for the effort.