It is best to measure your old cover if you have it available, otherwise please read the instructions carefully.
Should I measure just the spa (acrylic), or should I include the wood rim?
This is a personal preference, however most folks measure just the spa acrylic, lip to lip. (See figure below). In some cases you may want to measure to the wood rim, only because it may look better to you and it doesn't create a "rim" for leaves and other items to collect on. It does not help one way or another with energy efficiency.
Another thing to consider is the cover skirt/flap, you will want this to hang STRAIGHT DOWN. So, if the lip of the spa is at an angle, make sure you measure the cover long enough to achieve this. You don't want the skirt sticking out by measuring your cover too short. Also, measure the skirt (3" is standard) long enough to cover the spa lip, or the spa cabinet rim. (See figure below)
It is always better to make the cover size a little bit bigger than your spa when measuring. If you make a mistake, and measure too small, the cover will not fit properly, will not sit flat and will not hold the heat in.
Measuring Instructions for all Covers in either PDF or Word format