I just looked at the track , nice circuit but smaller than it appears , the front straight coming out of the kink is only 500 metres.
There are a couple of first gear corners and a bunch of 2nds.
I would try this, get your gear ratio set so you are exiting the 1st gear corners with enough room to shift to 2nd after corner exit and see how long you can stretch 3rd, you want to be accelerating hard in 3rd until turn in ,so third would still have to be getting towards peak RPM's.
If for some reason you top out in third and have to do a short burst in 4th you will lose time so you need to lower the gearing, if you are lugging the engine in the first gear corners then shorten up the gearing and spend more time accelerating in 4th on the straight.If you are not lugging in 1st then I would raise the gearing so you dont clip 4th on the straight.
Looks like a three gear Ducati track or a four gear Kawasaki track , I would try 16 t in the front.