How to transform ankle_pitch, roll to motors low_foot, up_foot?

[Slavoch]

Dear colleagues.

Firstly would like to say I really enjoy your project and robot, great job.

currently stuck with robot-sim frame transformations. real robot has closed kinematics chain on the foot. But in simulation there is pitch roll angles instead. As far as i get due to leverages on the foot low_foot, up_foot motors form reduction gear. Can you please hint how to do that?
I suppose to get something like
pitch = gear_ratio1 *low_foot - gear_ratio2 * up_foot + const1.
roll = gear_ratio3 *low_foot - gear_ratio4 * up_foot + const2.