I dont think that measure is a realistic tool as that 37.5% (300/800 = ~ 37.5%) of circulation sales implies everything in jump needs to reach 675K sales to be regarded as a success. That is already an amount that according to 2017 overall year data, only the top 29 manga volumes in JPN reached that number, so using that cut off, youre basically calling every manga in jpn as selling below average or Much Below Average. Only One Piece, Haikyuu, and HeroAca from jump had volumes over 675K.
The circulation also assumes every store that gets shipped the copies is selling the copies, the copies are being read, and people reading them are reading every single manga in the magazine each week. It is unclear or unknow if people dont just get a copy, read their one or two series and pass it on (put it on the shelf) or whatever else can be done. So I dont think measuring success by portion of magazine sales is a realistic, viable, or sensible tool.