H2O2 is traumatic on sinuses - use a Neti Pot with salt & tea tree oil instead

Hydrogen peroxide will burn and damage your sinuses at any percentage and is not worth the agony.  I know, I've done it.

The very best method of sinus clearing is to use a Neti pot (they are very cheap) and add from 1/2 to a full teaspoon of sea salt along with a drop of tea tree oil.  That will produce the most gentle and longest lasting sinus cleanse you can get.  Many formulas suggest a full teaspoon of salt but I've found that to be too abrasive and I've found that sea salt is the most natural feeling that you can use.  You can also use more tee tree oil if you feel that one drop isn't enough but the main point is, start out cautiously until you find your own level of comfort.  Water without the salt will be very uncomfortable so it's a requirement.

I've used the above method for bad colds and just plain sinus problems and it was recommended to me by my own medical doctor.