Listen to the bike.
Simple as.
The Drag has 3 sounds...
1/. Rumble grumble, you changed up too early. You might even find a slight juddering.
2/. Revvy revvy, I'm nicely in the powerband. Bike feels charged and powerful.
3/. Screamy screamy, feckin' change up!! Bike starts to struggle.
Sit there in Neutral and rev the bike up slowly a few times. Slowly roll the throttle and hear what the different rev ranges sound like. They will be about the same sounds, but quieter when you're moving along.
Now go out and ride some.
Listen closely to and try to feel what the bike is doing. When accelerating, you want to change up just before you reach the upper end of 2/. above. When slowing, drop when it reaches the burbly 1/. sound.
Eventually, you'll know what gear you're in by your speed and engine sound.
Every bike is tuned slightly differently, but you will eventually find the speed range where the powerband works best.
My up-shift points are generally: 0, 20, 35, 45, 60, 70.
I may shift up sooner, such as to 3rd at 30, if I am to remain at that speed for a while, but I may stay in 2nd to get more power for building speed further.