I have 4 halogen lamps rated at 240v 50w and wanted to put them on one dimmer switch, I noticed that the dimmer switch is rated at 400w but only 200w for halogen light, so some quick maths tell me I have 200w so is it ok to use the dimmer switch being that it is at the maximum limit?
The fact that you are hitting the limit and not going over it is not a problem. All devices include a tolerance its not like you are going over by a few watts. Don't panic just you are on the safe limit