OK, while NOT an aerodynamicist, my observations would suggest the following;
The hinge line of a trim tab needs to be regarded as the point of leverage that the tab has over the control surface, so the further aft the better.
At the same time, the area of the tab, as a percentage of the total control surface area will dictate the amount of deflection required of the tab to have an effect.
The tab you depict in your drawing would be considered 'adequate' but would require a lot of the 30º deflection to work and would cause a bit of 'Trim Drag' at it's edges near the fixed parts of the elevator.
A better solution would be to push the hinge line back to 20% of chord and make the tab incorporate two ribs and maybe even include the inboard section as well.
Have a look at a Cessna 150.....
Just my thoughts,