The 100Mhz he is losing will not be worth it.
1:1 ratio is for stability.
Run Prime95 Blend test for 8 Hours (Or overnight) and check for errors. This will tell you if you are stable or not. If you are stable, 5:6 will be fine. If not, 1:1 maybe fine but it does differ from Chip to Chip and Mobo to Mobo.