I would recommend that for named resources using a Max Units value of 100% for a full time person works, since this is used when Microsoft Project calculates an overallocation. Example Fred is 100% Max Units, he will be listed as overallocated when his assignments exceed 8 hrs per day by default. When I am using Generic resources I will set the Max Units to 0%, this will display the total capacity this is needed based on assignments.
This contradicts Microsoft’s stand on this issue. I quote the following from their MS Forum note. “Use Generic Resource set to 100% Max Units when you are not sure which named resources will eventually be assigned to tasks.”.