I’m surprised that you’re having problems reaching and maintaining a 225 temp on you CharGriller. How many chimneys are you starting with?
I usually start with 2 chimneys worth of coals (a mixture of briquettes and lumps). The temp climbs up very quickly to 225 and stays there for at least 2 hours. In addition, I rarely have to add more than 1 chimney worth of coals every 2 hours of cooking. And I often use the lower grate to house my coals unless I’m smoking food for over 8 hours, then I will use the upper grate.
A friend of mine had a problem with temp control on his CharGriller, when I went over his house to check it out, I found out that he did not remove the perforated metal plate that separated the 2 chamber prior to installing the side firebox. Thus he was not getting enough heat into the main chamber. I’m sure you did not do that, but I thought I’d mention it for future reference.
Try to line up the lower part of the main chamber with foil. Also leave the charcoal tray inside the main chamber during smoking. The foil and the charcoal tray will act as insulators and deflectors.
These pictures show how the temp on the led gauge and a gauge located on the opposite side of the firebox is equal and around 225 degrees. These pix were taken shortly after adding 2 chimneys worth of coals to a cold grill. The coals were placed on the lower grate in the firebox.
Please let me know if you’re still having trouble reaching that temp