The correct orientation of the cartridge in the Grohe pressure balanced valve depends on the vintage/model number of your valve body. Two different valve body designs were issued and the cartridge orientations differ by 90 degrees. My information here pertains to the cartridge installed in the 35 250 rough valve body only. This is a 3-port valve body without a diverter, or a 4-port body if it has a diverter. Its normal orientation would have the hot water input at the left (facing the valve stem), the cold water input at the right and the shower output at the top. The valve body itself has an H and C embossed on its surface. The H port should be facing the Hot input and the C port should be facing the Cold input.
The valve operates by simultaneously sensing and comparing the direct input pressure from these inputs and balances the pressure.
For Four Port Valve Bodies (which have a diverter) the principle of operation is the same. Match up the H and C markings of the pressure valve with the Hot and Cold Inputs. Be careful not to wrongly match them to the diverter plumbing, which runs at 90 degrees orientation. Usually, the diverter ports are vertically oriented because the shower head is above the head and the diverter valve is at waist height making therefore a vertical line. The Hot and Cold ports will then again usually be on Left and Right, respectively.