I'm seeing the same problem everyone else is. Casters, supposedly so skilled that they automatically succeed in casting any spell they know, are suddenly worse than a journeyman mage when faced with a particular adverse condition. This violates common and dramatic sense, so we have to fix it.
My suggestion is to use the different levels of penalties for RSR to represent how well the caster knows a particular spell, from -1/5 AP for a spell they don't know very well, all the way down to not requiring a roll at all for a spell they know very well. If the caster is in an anti-magic field, he casts the spell at the combined penalty of the field and the spell AP. If he doesn't normally make a roll (the spell doesn't have RSR), the AP of the spell doesn't add a penalty. This does assume that all casters will take the magic skill because some of their spells require a roll.
Another option is to simply require all spells to have the RSR Limitation, and handwave away the roll if the caster is sufficiently skilled (adjusted roll of 19-, or something like that). When they get hit by anti-magic, their chance of success drops below the threshold, and they have to roll.