Resize rotated rectangle with skew

Question

can you please tell me how to calculate the compression of a rectangle in svg. let's say there is a rectangle with x0, y0, w100, h100, r45, it is in an imaginary box, I have reduced this box by 50px, and how do I get the new dimensions? It seems simple, but I've been puzzling over it for half a day, at first glance it's simple trigonometric operations, I understand that the top side is the hypotenuse, and the catenary can be calculated using an angle, but how to calculate it correctly, because here you need to skew, and here I don't know how to calculate like r and 90 - r, but I can't put it all together, I don't have much knowledge on this topic, and I just recently started to get interested in it. I would be grateful if you could help me or give me links to resources on this topic. Thank you