The maximum step down is proportional to the linear mass density change.

I can go through the math but the short cut is to multiple the diameter of the larger section by 2 and then divide that by 3 to get the smallest diameter tippet you can tie on. For example, lets say the leader ends in a 2X section which is a diameter of .009". 2X9=18, 18/3=6, .006" or 5X is the smallest tippet you can tie onto the 2X.

You will read that you should not change more than .002" (2X sizes) at any leader know but that is pure garbage. The maximum change is related to the mass and stiffness of the thicker turning over the thinner section and a fixed "2X" difference is a different change in mass when 1X is tied to 3X (.002/.01= 20% change) vs a 5X tied to a 7X (.002/.006=33% change). So the relative change in diameter from 5X to 7X is 1.66 times that from 1X to 3X.

A fixed 2X (.002") change in diameter leads to different relative changes in mass a the knot.

The maximum change is a 1/3 change in diameter which the multiply by 2 and divide by 3 gives you. A 1/3 change in diameter is a 50% change in linear mass density. You can change up to this amount at the knot and the energy transfer will occure smoothly. Of course you can always change less than that amount.