Strange has absolutely no clue how to calm down someone who's in the midst of spasming in pain. He especially has no clue how to calm down someone when his only option is 'talk them down' and not anything he can do with his magic. He gives Herbert a panicked little 'how the hell am I supposed to do that?' sort of look before he starts talking, making his voice loud enough that Ignatius can hear him.

"Ignatius! Focus on my voice, everything's fine. I'm sorry I got you into this mess in the first place, but you're safe now. Things will get better, just focus on my voice and stay calm!"

And, unless Herbert or someone else tells him to stop, he'll just keep rambling half-formed apologies and repeated insistences that Ignatius just calm down and focus on Strange's voice.