How about using the distance from the center directly?

Adapted Code

1. float d = 100;
2.  
3. void setup() {
4.   size(400, 400);
5.   smooth();
6. }
7.  
8. void draw() {
9.   background(255);
10.   ellipse(width/2, height/2, d, d);
11. }
12.  
13. void mousePressed() {
14.   d = 2*dist(mouseX, mouseY, width/2, height/2);
15. }
16.  
17. void mouseDragged() {
18.   d = 2*dist(mouseX, mouseY, width/2, height/2);
19. }

oops, forgot to add a important detail (and like that is how i had it elsewhere).

It is much easier with a touch screen because you have a start distance you can then increase or decrease the distance by moving the fingers apart to bring them closer together. With a mouse you start with a separation distance of zero (mouse pressed) and the distance can only get bigger.

What you need is some reference point (touch 1) as the start and then when the mouse pressed this indicates (touch 2). The distance between them can be considered as a zoom factor of one. Now as the mouse is dragged the ratio of the new distance to the initial distance is the zoom factor.

The code below uses the screen centre as touch 1 but the top left corner is also good if not better., to try that uncomment line 11

1. int touchX, touchY;
2. float initTouchDist, currTouchDist;
3. float currDiameter = 20;
4. float initDiameter = currDiameter;
5. 
   
6. void setup() {
7.   size(400, 400);
8.   smooth();
9.   touchX = width/2;
10.   touchY = height/2;
11.   // touchX = touchY = 0;
12.   ellipseMode(CENTER);
13. }
14. 
    
15. void draw() {
16.   background(255);
17.   ellipse(width/2, height/2, currDiameter, currDiameter);
18. }
19. 
    
20. void mousePressed() {
21.   initTouchDist = dist(touchX, touchY, mouseX, mouseY);
22.   currTouchDist = initTouchDist;
23. }
24. 
    
25. void mouseDragged() {
26.   currTouchDist = dist(touchX, touchY, mouseX, mouseY);
27.   currDiameter = initDiameter * currTouchDist / initTouchDist;
28. }
29. 
    
30. void mouseReleased() {
31.   initDiameter = currDiameter;
32. }

thanks, your my goeroe :)