It isn't quites as simple as splitting the difference. Here's how I did the figuring to make sure things would work for my NE Dory.

Starting at the forward seat position (smallest beam), and assuming you (or others) don't desire to be forced to row with overlapped handles, plan to set the covers/collars as far outboard on the oars as possible at this position.  That will mean leaving about 1 inch between the two handles when the oars are horizontal, resting in the locks.  I like that full inch to prevent a big ouch! on your thumb if you ever have it wrapped over the end of the oar.  With the oars held in this position (and centered), then mark where the inboard side of the oarlock touches the oar.  Then, to give a little "breathing room" make another mark just 1/4 to 1/2 inch further inboard than that.  That line will mark where the OUTBOARD edge of your oar collar needs to end up.  So the INBOARD edge of the oar wrap and oar collar are actually about 1/2 inch further inboard than that mark (based on the width of your collar material).

Now, depending on the length of the wrap on your oars (probably 8" or so), double check that the wrap will provide protection when oars are placed in the center rowing position.  When you pull the oars "in" towards the centerline to again get the handle ends about 1" apart (my standard rowing spacing) you'll probably find the oar resting in the lock somewhere about mid-point on the protective wrap, with the collar about 4 inches inboard of the lock.  It all depends on the actual beam change difference between the two positions on your boat.  You're just checkig that the wrap length is long enough to cover this position, if not you'll need to use a longer wrap.

I hope this is all understandable and helps. In a nutshell you're just making sure to get the collars far enough inboard to support the narrow beam position, but no further than necessary, thus being sure the wrap also supports the wide beam position.

   Thanks for the detailed explanation.  It makes sense.