We remembered! We got 2 ceiling panels up in 1 bedroom. We had hoped to get the entire ceiling in 1 room done but ran into a little bit of trouble getting the length just right. We had to trim one panel three timesand the other panel twice. But they look good once we got them up.
A trick I use to get very accurate inside measurements is to use a 1x4 or similar butted up against one wall and measure from the opposite wall to the edge of the 1x4 then add the length of the 1x4. You can get within 1/32"

JT