The nature of their abilities are different as well. Madam web will see him getting shot by a gun... Midnighter could dodge it and kill the guy but if her vision of the future is due to midnighter being poisoned/slowed down/whatever reason that would allow him to be shot then midnighter will be shot. Comparing is a little hard.. But I'll try lol.
A little unsure of what your really asking but I'll give it a go.
I don't know 'everything' about madam web but she has Prescience and gets glimpses of the future while Midnighter has a battle implant/combat computer in his brain that helps him calculate Nearly every possible result giving him Precog. I don't think madam web can cancel out midnighter but I'm sure midnighter can mostly likely overcome madam webs predictions given circumstance and access to his own ability.