Less is Best!
The general fit you will see from most companies is to build in extremely aggressive retention on their IWB holsters. We offer retention adjustment on all of our holsters if this is your desired preference as well, but it is not necessary to death grip your gun in the holster, and dependent on the mount used with the holster, can actually lead to an unsafe condition with the firearm if it doesn't release off of the holster when drawing.
Generally, retention on IWB holsters will be increased by your gun belt and clothing. Keeping this in mind, you will want to keep your retention lighter than normal when the holster is not being worn, and you will want to only test your retention while wearing the holster with your desired gun belt setup.
This will allow your clothing and gun belt to add the additional retention without making it difficult or unsafe to draw the firearm.