Hi Richard,
The curve isn't linear, but that's not even the worst part of the problem. As the choke core magnetises, there is some residual field which results in a different curve as the current is coming down again.
There are a few references out there, however this looks like a good page to get an idea of the issue ->?
If you have a look through that, you'll see why I've always been reluctant to pursue anything more than an idealised inductor.
Regards,
Duncan
