In general motoring in the centre green section, usually a fraction up from centre. As long as you're travelling above about 20mph the cooling system should cope OK, even in hot weather. The 100º temps some places are getting will put it to the test though. No harm if it gets into the next green section if the engine is working hard in hilly country but it should come down again pretty quickly once you get back on the flat or easier driving. After a hard run, if the temp rises after you get back into town, check the condition of the rad cap/caps. A bad cap under these conditions can cause boil over. As the gauge is non electric it's normal for the needle to rise after you switch off the ignition and the latent heat raises the temperature of the coolant in the block. It comes back down quite quickly.
All the above assumes that the rad, water pump with properly adjusted belt, are good and the system is properly bled. New car, lots to check. Good luck from another wagon owner, in the UK.