As you have mentioned that you were getting good average around 50 and then now for some reason it has fallen to 35-38kms/lit. Bro, can you remind yourself what changes you have done recently to your bike, like Fuel station change, air filter change, mixture setting, route change, pillion riding, average riding speed, tyre air pressure, heavy tyre, wheel jam...etc
Have you checked battery condition because I have seen in GS-150 if battery is not good then fuel average marginally decreased.
For mixture setting, I have observed my mechanic incase of 150 and my Sprinter. He tighten (clockwise) the mixture screw to limit and then make 6 or 6.5 rounds of screw anti-clockwise. This way I use to get optimum mixture which is not to lean not too rich. Orange/brown plug will result in.