Yes, it is.
It is dependent on the thickness of the glass filter.
The mechanical backfocus is increased by approximately 0.33 times the thickness of the filter glass that is placed between the flattener/reducer and the camera sensor.
For example, if your filter is 2 mm thick, you need to add an extra spacer that is 0.66 mm in between the flattener and the camera sensor.
The factor of 0.33 comes from the refractive index of glass, normally 1.5 -- i.e., light travels 1.5 times slower than speed of light in vacuum. If you want more precision when the refractive index of the glass used in the filter is very different from 1.5, you can use ( ri - 1 )/ri as the factor to multiply by the filter thickness, instead go 0.33, where ri is the refractive index. For most glass use in filters, 0.33 is accurate enough.
The extra spacer can be in between the filter and the flattener, or in between the filter and the sensor, it does not matter.
Chen
