Got a new toro timecutter ss 42 inch zeroturn mower, checked tire pressure, rear tires ok, front tires say to fill to max of 30 lbs, have13lbs in back tires(they say ,max 15lbs)front only have around 15 lbs, kind of hard to get a tire gauge on the front ones, is it ok to leave them at 15 lbs or should I add more.