Starting with a couple of assumptions - that the slot is on the driven part and it is colinear with the radius, and that the driving part is the pin wheel - you can plot a line that reflects the ratio of degrees of rotation between the input pin wheel and the output slot wheel as follows. Looking at various points along the path in (and/or out) of the slot, the line that is perpendicular to the pin-to-axis radial line on the pin wheel becomes the Hypotenuse of a right triangle while the line that is perpendicular to the slot on the slot wheel becomes the Short Leg of a right triangle. If you divide the Hypotenuse by the Radius of the input pin wheel (pin center to pin wheel axis) you get an input distance A. If you divide the Short Leg by the Radius of the output slot wheel (pin center to slot wheel axis) you get an output distance B. This A:B is your distance/torque ratio for that pin position of your system. Using this formula you will get Infinity to Zero when the pin is entering at a right angle to the slot and the appropriate other ratios in the other pin positions. By always dividing A by itself to get 1 and dividing B by the same number, you can get an "apples to apples" data set for plotting your curve for how the input rotation compares to the output rotation. By calculating the ratio for a given set of pin positions derived by incrementally rotating the input pin wheel say 1 degree (1, 2, 3, 4, 5 degrees, etc.) and plotting all the resulting 1 to ### ratios you can create a curve that reflects the changes in distance/torque.
This is a fundamental approach that will give you a good feel for how and why things are happening the way they are.
