Do I see right that you only want to use two legs?
At 13 cm (about 5") they are not wide enough to support a 50 x 50 cm (20 x 20") table without extra support on the ground.
You do not need a computer program or phone app for this, people have made tables for many years before they even started with measurements, even less computers.
Place your legs on the floor, in the positions you want them to have when the table is made. (Straight up I thinkand try a few positions.) Put a clamps on the bottom of the legs to help them stay up, if needed.
Now put your top on it and balance it. It should stay there without you holding on if you want to have any hope of it working. The more to the sides your legs stand, the more stable the table will be side to side.
But the weak point will be front to back. You will have a very big area which is not supported well enough, so when the table is made with the legs in that position and you lean on the front or back of the table, it will tip over.
The whole of the table will do what the table top does when not fixed to the legs in that position.
To use these legs, you will need to add spreaders to the bottom of the legs, like the clamps I mentioned above.
The ends of the spreaders or horizontal parts will be the actual 'legs' in the bit I wrote below (Which I wrote expecting you to use 4 legs.)
The distance at the top does not matter, as long as you can place the legs at any angle. If you are restricted to straight or a limited amount of angle, your placement at the top needs to be adjusted to that.
The more at, or sticking out of, the corner of the table in all 4 corners, the more stable the table will stand.
I have a home made stool/mini table where the bottom of the legs are no farther than 20 cm apart (about 8 inches) but the table top is only 25 cm (about 10") in diameter.
For a 50 x 50 cm table (20 x 20") I would want the legs to come to at least 10 cm inside the corner. It can be attached on the corner and come straight down, it can be added in the middle of the table and stick out to almost the corner.
If you want the legs (4) or spreaders to be in the middle of the side of the table, they need to come out to most of the outside of the table to give enough support in the corners of the table.
You will also need to fix the top to the legs in such a way that the top is connected very securely, the more so, the more to the middle of the table you attach them.
Three legs would work, but four much narrower legs will be much less visible and give a nicer table in the end.
Two legs attached to a ring to stand on will also be sturdy enough, as long as all connections (to the ring and to the table top) are good.