Going to make my first all grain tomorrow


I've decided to go for batch sparging and i'm using Daab's batch sparge calculator. I want to make a 6 gallon brew but my Mash tun only holds a max volume of 28 litres. I'll need 5.5kg of grain (which I reckon will occupy 11 litres by volume) and 13.75 litres of water for the mash. After mashing I need to top up with 8.95 litres of water-hence the problem -it wont fit, I will only be able to get 3.25 litres in in for the top up

I could make my life easier and just do 5 gallons but I'd rather stick with a 6 gallon brew. Is it OK to add more water (the remaining 5.7 litres) as the level drops on the first run off? or what are the other options?
Cheers
Russ