How to know how many tablespoons of seeds correspond to 1 table spoon of its ground form?
It depends on how accurate you want, but based upon the book On Food And Cooking: The Science And Lore Of The Kitchen by Harold McGee, this is not possible to be 100% accurate. This is probably too accurate for your needs, but as you didn't state how accurate you want, I'll only explain why it can't be 100% accurate.

Measuring by tablespoons or the like doesn't take into consideration the size of the food on the spoon (the seed in this case). For example, not every seed has the same shape, size or mass. So when grinding to a powder, you're not working with consistent seeds.

This means, if you take 1tbs (or ever better, a cup) or seeds and then weight it, and then take another measure you'll have different results. 

The problem with powder, is it depends on how fine you're going. The finer will allow you have more powder in the same space compared to something 'grated' a little thicker. There is also the issue of air which gets into the powders which will have some factor.