Im not the king of vents but try parting line vent the entire cavity if it is flat. It depends on how flashy your material is. If its very flashy then .0006 might be necessary--if its low density something then it may not flash hardly at all and you might go deeper such as .0010. then just do a groove with a ball end mill maybe .020 deep that goes into that undercut area and run the groove to the outside..with the groove coming up maybe .100 from the edge.. Also when molding and having vent problems dont forget the tape and shim trick between the platens to space them apart.. I am a big believer in venting the entire periphery of the cavity even though it gives alot of people the willies. A good rule of thumb is that when venting that you probably need MUCH more.. keep in mind that a true full periphery parting line vent gives not less then TWENTY TIMES the venting....at the same depth. If .001 will not flash--then there is no logical reason to be afraid of venting the entire cavity. This proves out in actual practice and not just theory..