if pctfree is high then basically we are wasting hard drive space since only 10% of block is used for inserts, of course if you consider that you will update the rows so often and fill the 80% up then it´s up to you but I think it´s pretty rare
I say 10% but it can be less since pctfree+pctused cant be more than 100 and if we set pctused 20 and pctfree 80 most probably we will face perfomance issues because the block has to be put on freelist and taken off free list all the time.

As for block overhead I think there is a formula in Oracle DBA Handbook