KN is correct, 1 bar below the top mark is the correct measurement. The reason you don't want to go to the top bar is the inaccuracy of the bar graph. Below 50% of a bar will show as the lower bar and 50%+ will round up to the next bar. So if your graph shows at the top bar, you may be up to 49% of a bar overfilled and the graph will say just full.
An easy way to drop the level is to remove the oil filter canister and dump the oil from it. That should drop you a bar on the graph.
