1L MSR bottle is about the same as 1 quart. I average around 42 MPG, so each extra liter or quart of fuel adds about 10 miles of range.
With that in mind, I don't carry extra fuel at all for rides up to 70 miles. For rides 70 to 90 miles I carry a
1.5L Fuel Friend container in my backpack. I prefer this over 1L MSR bottles for both form-factor (size), capacity, and ease of pouring without making a big mess.
Here's my 1.5L Fuel Friend bottle in my 16L USWE pack with tool pack underneath and 2L Camelbak bladder behind it:
For rides longer than 90 miles I carry either a 1 gallon Rotopax mounted on a Nomadic Cycles rear fender rack, or a cheap 1 gallon plastic gas container strapped on my
Bill Dart fork-mount chainsaw rack (without the saw.) FWIW the only time I've ever need that extra fuel was for other bikes in my group. After 80 miles of singletrack I'm usually tuckered out and ready for shade and a beer!