Shower waste trap

Hello
Have looked through past posts to try and help myself with this but with no joy. My shower trays (already installed and tiled) waste hole doesn't seem to be completely flat. Waste trap doesn't sit flat. No gap on one side and 2-3mm gap on the other side. Have sat other waste traps in hole with same problem. Shower tray not yet used (brand new). I can see generally it is recommended to use a thin smear of silicon to help seal. What about "Plumbers Mate". Is it still used in the trade? Is it better than silicon in the long run? Is there a better brand to use? Or should i use silicon? If so what is the best one to use to get good long life as i am determined that this shower will not leak?
Many Thanks

plumbers mate is anything but. That stuff sticks to everything but the place you want it to go besides which it's far from ideal in this situation. Decent bead of silicone will do the trick wipe the excess straight away with CLEAN cloth dipped in white spirit, give the selant a day if you can to go off before testing

Have now found out why chrome flange would not sit down flat in shower waste hole. Builder/Plumber when they built our extension put the 40mm waste pipe in flooring void (upstairs) then installed shower tray from above. By this time they had installed ceiling directly below. Must have cut hole in chip board flooring, got hold of the waste pipe, cut it off to line up the shower waste trap with the shower tray waste hole and assembeled etc. from above.
I have now cut a hole (150mm x 200mm) in ceiling directly below waste trap. (Luckily it is to the side of that room.) The problem was that the hole cut in the chip board was not big enough so that meant that the closure nut on the trap exit when pulled up from above would hit on the underisde of the floorboard. As i tightened it up from above it would not come up to locate properly on underside on one side and would kick over.
Have found half decent looking white plastic access flap to put into ceiling hole. Looks neat, but have not put in yet.