You should use inverse kinematics equations for teleoperation/derive your robot:
The best way to the deriving curve is use from inverse kinematics equation and the best way for turning 90 degree (Both wheels move, but in opposite direction until 90 degree (this reason to your robot's dimensional )) which that handled by inverse kinematics too.
r is wheel radius,
d is robot width,
v is linear velocities,
w is angular velocities, and
thetaL are left/right wheel RPM that you must get them.
And before these, you must obtain/solve
w from using wheels encoder.
In additional, check this post: calculating differential drive robot ICC position