Baseboard is measured as the room perimeter minus doorways, then divided by the stick length you can actually get home. Twelve percent waste is normal because every inside and outside corner is a mitre, and mitres get cut twice.
Match stick length to wall length
The goal is one unbroken piece per wall. A wall longer than your longest stick needs a scarf joint, and scarf joints open as humidity changes through the year.
Standard lengths are 8, 12, 14 and 16 feet. Buy for your longest walls first and use the offcuts on short walls and closets.
Corners eat material
Outside corners are mitred at 45 degrees and use extra length on both pieces. Inside corners are best coped — one piece butts into the corner and the other is cut to its profile — which wastes less but takes longer.
Coping is worth learning. Mitred inside corners open up visibly when the framing moves; coped ones stay tight.
Walls are never square
No corner in a house is exactly 90 degrees. Cutting every mitre at 45 and hoping produces gaps at every corner.
Measure the actual angle with a bevel gauge or an angle finder and split it. Or cope the inside corners, which sidesteps the problem entirely.
Order of operations with the floor
Baseboard normally goes on after the floor, not before, and it sits on top of the finished floor rather than on the subfloor.
The exception is carpet. There, baseboard goes on first with a gap underneath, and the carpet is tucked into that gap. Get the order wrong and you either trap the floor under the trim — which stops it expanding — or leave a gap you have to cover with shoe moulding.
If you are unsure, install the floor first. It is much easier to scribe trim to a finished floor than to work around trim you already nailed.
Shoe moulding and quarter round
Almost no floor meets a wall cleanly, and shoe moulding is the small strip that covers the gap between the baseboard and the floor.
It is not the same as quarter round, though people use the names interchangeably. Quarter round is a quarter of a circle; shoe is taller than it is deep, which looks more deliberate against a tall baseboard.
Either way it is a second linear-foot calculation at the same perimeter, and it is nailed to the baseboard, not to the floor — the floor has to be able to move underneath it.