Missed this question from last year, and just now saw it, now that I'm thinking about lawns again, since the FIRST Robotics activity that usually consumes all my free time in March and April has been cancelled for this year due to COVID-19.
Anyway, to measure a half a Tbsp, I use the small end of a standard "Miracle Gro" measuring spoon (the kind that comes in every container of Miracle Gro) like the below:
The big end is 1.5 Tbsp (tablespoons). The small end is 0.5 tsp (teaspoons).
There are three teaspoons in a tablespoon. So, three half-teaspoons totals one half-tablespoon.
So, to get 0.5 Tbsp, I use three leveled-off scoops of the small end of the Miracle Gro measuring spoon.
Any accurate way that you do it is fine, but that's how I do it.