Hopefully this will help some others...

I believe there is a simple (if not tedious) way to GUARANTEE you will have a certain number of trials where a stimulus appears on the Left or Right as well as if the trial is 250, 500, etc. milliseconds.

The only way I know to do this involves using the same WGR value for each trial. Basically, you need to create the number of trials you will need along each dimension (e.g. 30 trials at 250 ms, 30 trials at 500 ms). Then, make half of those trials have a dot appear on the Left, and half have the dot appear on the Right. By having each possibility represented as a permanent trial in your input file, you will know it will be displayed. Moreover, randomization using the WGR column will ensure the presentation is random (and thus order in the input file does not matter).

Additionally, in order to BOTH randomize whether a stimulus appears on the Left or the Right side of the screen, AND to ensure that you will know if the participant responded correctly or not (using the Time column), you will need to code each trial ahead of time and WGR randomize the trials.