It's a bit kind of tedious. The formulas are detailed here:
More power to your Barlow by Nils Olof Carlin
Using a barlow vs screwing the lens group directly onto the end of the BV will result in less magnification but usually not a lot less. That's because the light path inside the BV is typically a fair bit longer than the barlow itself.
Easiest way is to not go too hardcore with the maths and the formulas but experiment to see what works and how well. (But by all means run some numbers but also let observation be your guiding light.) Defocussing and moving the scope around is a good way to check for undersized field stops (biggest issue with BVs).