Okay, let’s break this down. Susan has 43 years of experience in interior design, which is a long time. Typically, senior-level designers or principals in their own firms can command higher rates. Considering her extensive experience and the fact that she's working independently, I'd estimate her hourly rate to be around $125. This is slightly below the average rate for a senior interior designer in the US, which can range from $150 to $250 per hour, depending on location and industry demand. However, Susan's 43 years of experience and her own firm's structure suggest a more modest rate.