It's always best to check with your doctor first for dosage on medications. When we have used medications, the doctor always goes by weight (mine are bigger than the age/weight ratios on the box, too). But 30 lbs for a 10.5 month old - I am impressed! :)
I would be sure your doctor knows which type of benadryl you have, children's or infant's (if they make both, I'm not sure). But if the box is saying 1-2tsp for a 6 year old, 1/2 tsp for a 30 lb baby sounds about right to me.
Sounds like your a little nervous about giving your baby the medication. (I'm always nervous about it even when I'm convinced it is absolutely necessary). I think I have heard of some natural remedies for allergies. Maybe some other moms will respond with info if they know about that. But if you're interested, you could try going to a natural food store that has supplements and homeopathics and talk with someone who works in that department. They may have something natural that would help your baby so you wouldn't have to go with harmful medications. Just a thought! I know you didn't ask! :)?