I have been told by more than a few women that they are afraid of hurting their partner when the spank or paddle him. Well... You could if you really lay into him. But let me give you a few pointers to help you feel more comfortable.
1. Always use a safe word. A safe word gives him the opportunity to stop you if you are hurting him. Some people will say that it gives him too much control. I say, “Hogwash!” It won't take you long to realize just how hard you can hit his upturned bottom. But in the meantime, if you are afraid of hurting him, it's a good idea,
2. Watch and listen! You know how much he normally struggles and how loud he yells when you are spanking him. Listen for that especially distinct “Ouch!” that you hear when you know you have hit too hard. As long as he isn't doing that, you're OK.
3. Hit the “Sweet-Spot!” The spot from where his bottom becomes his thighs and about halfway up (six inches?) his bottom. That is the “Sweet-Spot.” Odds are, you will do less damage hitting him there. Too high, you risk hitting his tail bone. Too low, and the pain will seem much greater because the thighs are more sensitive. (I know I will catch a lot of flak for that one, but I know I am right.)
Always start out slow and soft. Allow yourself to build up to the really hard hits. If you use your hand the first few times, you learn how it feels because you will feel it, too. By starting out more slowly, you will learn more quickly just how much he can take. If your goal to punish him so that he won't soon forget, use fewer, harder smacks. If your goal is more of a play thing, then start slow and let the pain build. Remember, if you can give pleasure while giving pain, he will be able to handle much more. His brain can't tell the difference, so start out by gently rubbing his bottom on a soothing, loving manner.
Another way to tell is to check your handiwork the next day. If it's not all purple, black and blue, then you certainly didn't hurt him. If it is, make sure there are no welts or blisters. These can be a sign that you went too far. By-the-way, if you make him take a nice HOT bath afterwards, you can be sure that the he won't forget too soon. After all, the longer the pain lingers, the longer he will remember it.
If the problem turns out to be your own feelings of guilt. Have a long and serious talk with your partner the next day. Ask him how it felt. Ask him if it was too much, or if he thinks he could take more. Many men enjoy spankings considerably. If your guy is one of those, then he will be honest with you.
I hope that helps those of you who were in doubt.