Sounds like a lesson that I would learn - lol. OBTW - putting old fashion "Rubber Cement" , the kind that kids use in school projects (folks outside the US can not find this so I am told), works fantastically as well. Just let it dru for about 30 minutes. Oh did I mention that it too needs to go on the bottom??
Since there is no heat involved, it can not damage the ruler and can also be pealed off easily when it gets yucky.