A PVC joint is an interference fit...i.e., the socket of the device is smaller than the piece you are trying to fit into it. (In the case of an inside fit, the flange is tapered, and gets tighter like a plug as it goes in.) When you add the cleaner and the cement, the outer surface of the pipe and fitting melt, and fuse together. In other words, it is often nearly impossible to dry fit pvc pipe and fittings...you must measure. If you did get them to bottom out, you might have a huge problem getting the apart to then glue it up. If the flange does not have a champher that would prevent it from bottoming out when you add the cement, it will go all the way down. If there is a champher, then you will likely need to cut the pipe back a bit. Look at the bottom of the flange - is the junction square or not? Also note that because of this champher, you must hold things in place until the solvent diffuses, or the fitting can ooze itself back off because of the taper and the lubrication from the cement. Maybe put a box of tile on it or something until it sets up. It's a good idea to use the screws on the flange to hold it in place. Keep in mind that it probably isn't the greatest idea to have the plastic the only thing holding the toilet in place...anchor the flange, especially if it is all plastic. You may need a diamond bit to make a hole through the tile. Some floor tile is really hard. A good tile-setter will often notch the tile so he doesn't have to drill through them later to set the flange anchors in.