The critical aspects of roughing are:
Getting to depth The tool needs to get to depth, and this can be accomplished by a zig-zag in Z (the default setting and influenced by Max ramp angle), by plunging, or by predrilling (see Pre-drill diameter and Pre-drill point).
Vertical step FeatureCAM cut depth is no more than 100% the tool radius (see Rough depth and Rough pass Z increment).
Horizontal stepover FeatureCAM moves over laterally a percentage of the tool diameter (controlled with Rough pass %,) as it steps across the feature.
Finish allowance controls how much material to leave for the finishing pass. By default this is 0.02. For a simple slot, Finish allowance has no effect.
See also How slots are manufactured and FeatureMILL 2D Milling algorithms.