Raising monthly rent after keeping it the same for 10+ years

[Karen Chaudiere]

Since I can't change my past behavior, I need to improve going forward. I have 2 single family residential rentals in Scottsdale, AZ that have stable tenants. Unfortunately, I didn't "manage my business" and have not raised the rent in over 10 years. Each is at about 60-70% of current market rates for the area. While I realize it might not be a good idea to attempt to raise the monthly rent $300 next time the lease is up, what IS a reasonable % increase to charge? - my goal is to increase the rent AND keep the tenants (they have taken very good care of the property and always pay their rent). 

Since these are out of state properties for me, I tend to avoid turnover.

Any advice would be great appreciated.

Thanks

PS I am increasing my RE investing in the greater Seattle area and planning my first purchase later this year