Graham,
The backfocus of the Microline 8300 is 16mm from the sensor to the faceplate. See here:
http://www.flicamera.com/microline/index.html
You must have measured to the top of the dovetail fitting on top of the camera which goes into the filter wheel or female dovetail adapter that you may use. Unless you have screwed a T thread adapter into it in which case that sounds about right.
I'd also check with users if 1.25 inch filters are large enough. Usually its 36mm. QSI cameras filter wheel is closer to the sensor and that's how they are able to use 1.25 inch filters. Even so 36mm no doubt has less vignetting.
Greg.