From empty, as you are changing the filter as well, you're safe to put in 2.5 litres straight off.
Then, start the engine for 20-30 seconds so it coats the engine, switch off, and then watch the window, the oil will slowly rise up.
If it's not enough
add a bit at a time, then start engine again, switch off, then watch the window again.
Repeat this process until the oil level is
in between the metal markers at the side of the window.
Make sure the bike's on a level flat surface on the centre stand though otherwise you'll get a false reading.
If you're thinking of changing the oil around the 4000 mile mark, you only need to replace the filter every 2 changes, it's not necessary every time.