Qualified business use for the purpose of determining the depreciation method doesn't include the personal use.  So in your example, you have 80% qualified business use.  Since that is more than 50%, the S Corp can use accelerated depreciation.  If, in a later year the QBU drops to 40% (because the S Corp owner has 60% personal use) then the S Corp will have depreciation recapture to the extent of accelerated depreciation.