LTSpice: Find settling time using .meas
You can use this command:

.meas tmp find V(o) when abs(v(o)-1)=0.01 fall=last


Alternatively, you can concoct something like this for a more "dynamical" approach:



I commented out the .step card so that the results are a bit more visible. This is just one approach. Note that this implies knowing the I/O step value(s). I suppose you can do that by simple subtraction, but you know what cases you have for that.