Does anyone know anything about gardening?

Not really, boards have a way of varying by fractions despite going through a profiling machine, you can only get approximate. Expect to cut down the final board lengthwise with a circular saw to fit. Cladding around the frame will tidy everything up. Presuming a width of 1.5m, 7 boards needed including cladding, adjust 4x2 quantities to fit within frame with a 33 to 46cm spacing between joists. Structural screws will remain at 100 as smaller packs don't exist. A good rule of thumb is 50 decking screws per square metre.

For anyone else reading this and wondering how to space supporting posts, my golden rule is a minimum of 2m between posts, anything much more and you will be building a trampoline rather than a decking.