If your shoulders leave the seat whenever you steer through a tight corner, start with the driving position. Move the seat and pedals before relocating the wheel. You’ll have a fixed place to work from when you check the reach to everything else.
Start at the pedals
Settle your hips into the seat and put your heels where you normally rest them. Press the brake as firmly as you would in the sim. Look for a slight bend in your knee at full application and check that your hips stay against the seat. Repeat with the throttle and clutch if you have one. If you’re reaching with your toes or shifting your hips, move the seat or pedal tray a little and check again.
Bring the wheel to you
With the wheelbase off, rest your hands at nine and three. Your elbows should have some bend and your shoulders should sit comfortably against the seat. Turn the wheel through the range you use in a slow corner. Check that your hands clear your legs and that you can reach the paddles from your normal grip.
Give the new position a short drive. Notice what happens during a heavy brake application and a corner that needs a lot of steering. Small adjustments are easier to judge when you repeat the same two situations. Once it feels right, record the seat slider position and take a photo of the pedal and wheel mounts. Those references save time if someone else uses the cockpit or you take it apart.