VECTOR forceVec; // our vector we will set depending on joy_force and use for rotation - global definition or local but not in a loop!
var forceThreshold = 0.1; // our threshold value, adjust this if needed
...
// in the function:
vec_set(forceVec, nullvector); // reset our forceVec

if(abs(joy_force.x) > forceThreshold)
{
    forceVec.x = sign(joy_force.x);
}

if(abs(joy_force.y) > forceThreshold)
{
    forceVec.y = sign(joy_force.y);
}

vec_to_angle(my.pan, forceVec);
my.pan -= 90;