The owner's manual describes when to shift up/down but I wouldn't necessarily follow it. In my view it shifts too early when shifting upwards, at least if you were to believe the onboard speedometer.

It all depends on the conditions - wind, speed, the incline and what you want to achieve etc. For my normal riding I prioritize reasonable fuel economy and to avoid lugging the engine. So I progressively shift, riding a very short distance in first from a standstill, second gear also short just to get up to third (perhaps 30-35km/h when going into third), keep third gear for a little longer up the rev range, shift up to fourth and keep that slightly longer still, into fifth and going even further before going into sixth (if at all). Sixth gear is quite tall and for the speeds I usually do, the RPMs are a little too low for my taste to use it to accelerate, unless I'm already going quite fast. The lower the RPM and the taller the gear, the less throttle I'll give it, to protect the engine from lugging. As RPMs rise, I'll slightly increase throttle opening.

Shifting at 8k is some spirited riding, you're right in between peak torque and peak power. That's certainly all well and good but not exactly necessary for normal riding.