I can tell you what I use
Hello,
I have a 3/4" thick BC fir 7 ply 48" long and 20" wide sitting on an old office desk.
I check its flatness using a 36" long steel ruler that I place in different orientation; lengthwise, crosswise, in both diagonal and any various orientation. If I cannot pass a sheet of writing paper 0.003" thick underneath the ruler with the ruler in both edges on the surface then I think it is flat enough for our purpose.
Using both edges of the ruler proves that both the ruler and the plywood surface are within 0.003" and that is certainly good enough.
Some may say we cannot push pins in that plywood. I grab the pins with 5" visegrip and hammer them in just so they are solid. I do not put pins through the balsa ( or wood of the model structure ). I put pins at an angle to hold the parts in location. Not many pins are needed.
If pins can be pushed by fingers then the material is too soft and its flatness cannot be relied on.
Now do whatever you are comfortable with. Just make sure you have a flat surface when needed.
Zor