This is how you could calculate without a gyroscope. How far apart are the wheels width-wise, from center to center? Assuming you’re measuring on wheels on an axle around whose center the robot is turning, half that distance is the radius of a circle on which the wheels drive. If you want to make a 90-degree turn, you want the wheel to travel 90/360 of the circumference. Code the motors so the wheels travel that distance forward or backward. Now, if your robot is not spinning around that point, the geometry is trickier, but the basic concept is the same. You’ll have to figure out the radius of the circle and how much the wheels are actually trying to travel tangent to the circle (since they’re probably not aligned with the circle like with an x-drive).