If on transit, you find you are a little 'light' for fuel for the return, you can consider:
1- reduce taxi fuel to less than 200kgs
2- possibly chose a closer alternate to reduce diversion fuel
3- read your ops manual, but you should be able to specify a closer destination with your original destination as alternate. You do NOT have to specify on the flight plan you have done this. Inflight, 95% of the time, you will find fuel in hand to continue to your original destination. It is really just a paper exercise.
4. Is your final payload going to be lighter than the estimated payload used in the return fuel plan?
The aim is to not have to connect a bowser to fill up with just 300kgs fuel!