Pre-boil water volume increase = grain bill increase?

My all grain setup, with it's inherent dead spaces and boil/cooling losses, require that I start with 8 gallons of pre-boil water (to yield 5 and a bit gallons).......instead of the 6.5 gallons of pre-boil water that many recipes say I should be starting with.

Should I adjust my grain bill somehow to account for the added water with any recipe that asks for less pre-boil water?

You should adjust your grain as needed if your gravity is low. If you have not brewed yet with this system, one can only guess. Some like to err on the high side and add a pound or so base malt if uncertain.

Don't worry so much about what the recipe says, you need to do what works for you. Starting with 6.5 to yield 5 seems overly optimistic and unlikely IMO.

If short on volume, more water....if short on gravity, more grain. If short on both, more of both.