What would be an appropriate rate for increasing monthly rent on my fourplex?

[Nicholas Sorace]

I have 3 units, they are on the older side but very well kept by the current tenants that I inherited. All units are in the $650-$675 range and average market rent is at $850. I would love to keep these tenants, but I also want to eventually renovate the units and bring them up to market rent. A renovated unit goes for about $1000. 

I was thinking an 8%-10% increase on a 6 month lease. Any thoughts on this would be greatly appreciated.