RMU states Brigandine as AT9 (on a 1-10 scale), above mail (AT8) and below plate (AT 10).
However, the description does not necessarily match the appearance of the Brigandine armour - the Brigandine is supposed to have *small* metal plates riveted to an outer layer (leather or cloth). The RMU description indicates "larger than hand-size" plates, and makes no mention of flexible layer or riveting - plus, it gives the roman Lorica Segmentata as an example, which significantly differs from the Brigandine (larger overlapping plates, no outer layer or backing). The actual Brigandine matches the Metal Scale (AT 7) description better, though not perfectly either.
Additionally, apparently, it was typically worn over another layer of protection, be it a gambison or mail shirt. Additional arm and leg plates completed the protection. I would keep the full set as AT 9 (RMU) and would push it to AT 15 (RM2/RMSS/FRP) - not quite as good as plate, but the next best thing. You would have to customise the manoeuver and defense modifiers, though.
Worn without the gambison or chain shirt, I would put it as AT 8 (RMU) and 14 (RM2/RMSS/FRP).