I think all table requests have to be made at the restaurant itself. Be pleasant and offer to wait. Being on time or early would probably help too.
For most restaurants, you can wait and make one call. I do this all the time and have never had a problem getting the restaurants or time I prefer. for our next trip I made PSs for the first half of the trip at 90 days minus but was up in the air for our itinerary for the second half of the trip. Just made those PSs today at a little over 60 days and got everything I wanted at the times I wanted. Oh, I couldn't get a MK dinner as late as I wanted but I know later times will open up closer to the date. So as of now, we'll be having dinner at 4:45!
There are exceptions to this of course. If you want to have breakfast at CRT you should call 90 days out first thing in the am. The other princess meals may be like this, too. If you will be dining on a holiday, call as soon as you can.