A common commuter clause requirement is that you have two flights available, one of which arrives 3 hours prior to your show, and the other arrives NLT than 1.5 hours prior to show. Both flights must be on an airline you can non-rev on, and must show seats available in the cabin the day before. If you apply something like this you should be fine.

On probation, I would want a third backup flight. It's unlikely that you would get fired for one missed commute if your plan was conservative, but if you do miss a show, be VERY careful after that (at many companies you might want to move to domicile).

If is looks like you might not make it, call your crew schedulers and let them know so they have a few hours to call a reserve. If your no-show causes a canx flight and you didn't warn them, you are toast. OTH, if you give them a heads-up, and they can cover it, you can probably just take over for the reserve pilot who flew your trip and maybe get away with a phone call from the CP.

A lot of this depends on your company and their attitude...ask around.