Hi,
Supplied fuel tank is 4L. I use one evoJet 180 turbine and for 6 minutes flight + taxi for takeoff and landing it consume 3L.
So, I have one litter for go-around. I had an extra 2L tanks and I have installed on my Hawk but I fil it only with 1L of fuel. So, I have 5L instead of 4L. I plan ti use this extra tank for smke oil in the future.
It is up to you what you want to do.