Usually when I drill MDF, I just eye up the screw shaft with different drill bits. Most of the time I use standard sheetrock screws, both #6 and #8... I'm not too picky. I select a drill bit that is just slightly smaller than the diameter of the shaft. Then I use either a countersinking bit or a 3/8" or so bit for the head. Go slow if you are using a regular drill bit for the countersink. It can easily grab and end up being a much bigger hole than you intended.
keep in mind that I usually don't use MDF for sub boxes. It'* more for woodworking jigs and whatnot but the principle is the same.