A tricky one as there is no straight line to work to. 2 options really.
1st is to use a flat mdf strip and scribe around the weather boards and no more gap fill.
2nd is to prefill the gap and then use paper tape on the flat right up to the weatherboard. Then run a bead of gap filler to prevent a hair line crack.
I would use the 2nd option for a nice clean finish that won't crack.
__________________ Do not use fibreglass mesh tape on butt joins, ceiling joins or fibre cement sheet products EVER GREAT PLASTERING TIPS AT | 
Weatherboard meets gyprock 
