Are you sure that your calculations are right?
Over the thumb I would say your thrust is a bit less than twice the scale reading + ca. the weight of the rocket (I didnt run the numbers, but your string splits the force to 2 fixations of the string (like a pulley) and your scale measures only one side, but I has to be less than x2 since your string is not vertical and so you got a horizontal part inside your scale readings (should be about x sin(angle of string)).
So you would actually be closer to 20-25kg thrust…
And you might have gotten @Mat wrong 