Just pencil mark through the toilet support holes at the base and then gently drill through the tile. Once you reach the wood use a smaller hss bit to drill a pilot hole in to the floorboards/sheet material below and then re-position the pan and use, I think memory serving correctly no.10 guage screws, length is depending on the depth of the toilet base. If you can get your hands on rust resistant screws, all the better. don't overtighten against the pan base.
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