With these it isn't so much setting it to an exact temperature. The minus side is basically "cool" the plus sign is "hot" so the closer to the plus, the more heating it will do, thus increasing the temperature of the tank and just observe it for a day and see what it keeps the tank at and then you will know if you need to put it towards the minus sign or towards the plus sign more.
I had to fiddle with mine to get it to a point where it'd keep it constant.
I'm not sure what you mean by removing the cap? Setting it up was taking it out of the box, putting it in the tank, adjusting the dial, plugging it in and waiting to see where I wanted that dial.