Hammer it hard to avoid any spacing.
How to frame a soffit roof.
The nailed board is called the ledger of the soffit s framing.
In this case the eaves consist only of the rafter tails and the spaces between their ends are left open.
Slide the square down the rafter and mark where it touches the top of the wall.
When hammering the ledger ensure that you do a thorough job of securing the ledger compactly i e.
Measure and mark that distance in at the end of the wall.