I assembled the spacers, OAG and camera and the best I can do is about 130mm. Which is better: a little too short or long back focus? Is 130mm good enough?
I asked such questions when contemplating my Edge 9.25 purchase, and was referred to this link
In a nutshell, unless you're using a ridiculously large sensor chip like a U16M (4096 x 4096 at 9 microns, or 52mm diameter), you don't need to worry about a few millimeters.
Your SXVR-25 has a chip that's identical to my QHY8L -- 26.x mm diagonal. That means that the furthest point from the center of the chip is 13.5 mm distant, thus the 13.5 mm line on the above chart is what matters most to you. As you can see, even within 5 mm there's almost no appreciable difference.
It's only when you vary by a huge margin with a moderate sensor size like ours, or when you vary by a smaller margin with a huge sensor size like 52 mm (where the 26 mm line in the chart applies, showing rapid curvature as you drift from the sweet spot of back focus).
That chart is for the EdgeHD 11", but the same principle applies to the other apertures. For our sensor size, don't sweat it. Get within 5 mm, and call it good. You did, so you're set.