You may want to read the following link on the back focal distance.
https://www.cloudynights.com/topic/6120 ... corrector/
If the Focus Distance to Sensor is 12.5mm, and based on the following, then you need about 97.5mm.
Spacing for the 0.63x reducer is really sensitive to the size of your imaging chip. If you are using a small chip 1/2" or smaller then you really don't have much to worry about and 100mm or 110mm is not going to make much of a difference. The issue in spacing will be most critical on larger APS sized chips, here you will want to measure from the center of the reducer element to the approximate location of the sensor and 105-107mm is probably the range you want to target however it could be different and you will notice the optimal spacing where you get the best looking stars around the edges. Now another consideration is that the C6 has a much smaller imaging circle than the C8/C11/C14 so realistically you will start to get a lot of vignetting once you use a sensor with a diagonal much over 20mm. So realistically you are really limited to 4:3rds sized sensors or smaller. But like so many things in this hobby there is no definite answer rather you have to experiment.
However, the following link states: Produces a corrected field of 24mm diameter. The back focus is 85mm, from the reducer’s back element to the cameras sensor. Based on this information, you'd need about 72.5mm.
https://www.bintel.com.au/product/celes ... 42e4af0916
So, what's the right answer - will likely require some testing. You should be able to do some daylight testing to see how much of an image is captured. I'm sure there's a mathematical calculation for the right answer, but it's beyond my expertise - and when all else fails, I try a little "trial-n-error" testing to see what works.
[Update - the range for the back focal distance seems rather large, but given the sensor size, you should be able to capture an image but not the full width.]
Your camera has the following sensor dimensions [Sensor Size: 4.8mm*3.6mm, with Diagonal: 6.09mm], so the image will will cover the entire sensor - but will let our ZWO experts opine on my assumption.
Let us know your results. If I've got enough extension tubes, may test it out and let you know. Unfortunately - only have the ASI1600, ASI120 and ASI071 cameras that I use with my William Optics Zenith Star 81 (which works marvelously for me).
Good luck, Jim
William Optics ZenithStar 81 & NexStar 6SE
Celestron AVX GEM 5.31.9200 with MC 7.15.8270
ASI1600MC/071MC/120MM w/ EAF & EFW
iPad Pro w/ SkySafari Pro 6