[Matt Ratcliffe]

Here’s method I use for relecting particles (especially with bounce): If your camera remains stationery, it could be easy for you to use if u want to get the reflection angle perfect. This is similar to Scott’s method but would retain the correct reflection in the z depth:
• Copy your comp
• switch off all layers except your camera and particles
• move you cameras Y axis to the opposite height by adding the comp screen height (ie, in 720P, if you camera is at default dead center, it would be positioned at x:640 y:360 z:default, reposition Y to 360+720=1080). Keep the point of interest the same.
• In the original comp, add the new camera comp, unlink the scales and make Y -100, switch blend to screen or add, lower trans if needed.
• From this point I reposition by eye, When using bounce u just line up the bounce points and it looks perfect, but without bounce I would just find the balance between the height of the relefction plane and the real particles.

Good luck with which ever method you end up going with!!