Aaron, as I understand it, you wish to plane a slight rebate around the perimeter of the panel. I do a lot of this when preparing drawer bottoms to fit inside slips.
A nicker is for use when planing across the grain, to prevent the shoulder spelching. As Warren mentioned, ideally the nicker needs to be a smidgen on the outside of the blade. The nicker should not be needed when planing with the grain.
As Jim mentioned, if the edge of the blade is not sharp, planing with the grain with lead to spelching. I suspect that yours is not sharp.
To aid in planing across the grain, what I do is mark the shoulder with a cutting gauge. Not only should this prevent spelching - as long as the blade lies inside the line - but you can use the indent to set the nicker-fence combination, and then check the blade set up.
Regards from Perth
Derek