I will try to throw my two cents worth into this discussion, and hopefully will not confuse you even more.
First, If the filters are said to add 1mm of backfocus, I would interpret this to mean that they will add to the total backfocus length so that instead of 85 mm your backfocus will be 86 mm.
Second: To your question about why the backfocus specification matters I would say that unless you are using a telecompressor or field flattener in your imaging train then the backfocus probably doesn't matter (as much) because as you stated, you will adjust the focuser until the image is in focus, and that is as good as you will get.
Take a look at the drawing below. You will see that there are actually two backfocus dimensions; dimensions X and Y. If you are using a telecompressor or field flattener, dimension X from the back of the compressor or flattener is critical and will determine whether you get pinpoint stars across the whole image, or if they are points in the middle and elongated at the corners. Just about every field corrector or compressor on the market will have a specification for the total backfocus, and if you have filters you will have to account for them in determining what the actual backfocus is. As Dale has said, it is better to be a little bit undersize and to make up any difference using shims. Once you have assembled your imaging train, the distance from the rear of the corrector to the sensor is fixed, but it will still be necessary to adjust the focuser to reach focus.
If you are not using a field flattener, then dimension Y is really what you are interested in because it will give you an idea of how far out the focuser has to be racked to achieve focus. You may decide that if you have to rack your focuser a long way that it is better to add an extension to your imaging train. In the sketch below you can see that I have to rack out the focuser on my Stowaway about 2-1/2". I am considering adding 2" thread-on extension in front of the flattener so that I do not have to rack the focuser out so far.
The worst case scenario is if you can not rack your focuser in far enough to reach focus.
I hope this helps.