I usually make my calibration boards based on the height of the object to be scanned.
What you have submitted as your calibration size seems to be correct to me. I think that if you make some different sized calibration boards, you will find what best suites your needs.
You may want to try making some patterns that are half the height of your object to be scanned and placing those on your 85cm boards. I think they will work to. Personally I have found that if the calibrations are about 1/3rd the size of the background boards or smaller, I tend not to get immediate recognition by the DAVID program of the points and have to end up doing it manually or adjusting the contrast/brightness settings. I hope this has helped you.