Change to standard calendar or change to resource working hours?

I oversee the timelines etc for 5 software developers who all work on several projects.

On the whole the decision has been taken to schedule them to work a 6 hour day (to take into account various factors that mean it is near on impossible to actually work a solid 8 hr day when developing). So for example if we quote a client '1 days work' I will be scheduling the developers for 8 hours - so this will span a day and a couple of hours. However, (typically this isn't always going to be the case) - as we do have the odd contract that has the developer on site for a specified number of days, so in this instance I would want the 'standard' calendar to apply.

My ideal is to have a resource sheet, then each project bought together on a master project - pulling the resource allocation through from the resource sheet.

My only confusion at the moment is whether I change the standard calendar on each project (or create a template to work from) showing this 6 hour working day, or if I should change each of my resources to show they only work a 6 hour day...?

Then what would I do in the instance where the six hour day doesn't apply?

hope that makes sense!

Caroline