How many souls, of what level, etc, does it take to recharge staves?
Simply put, if an enchantment was placed on a weapon with a grand soul gem, for example, it takes a grand soul to fill it when empty. So really all you need to do is go into your inventory. Since you should have a few of each size from petty to grand, get the staff to empty, and then go through seeing which soul gems will fill it with a single soul. Start from a Grand Soul and go down in size until the soul gems do not completely fill the bar, which ever soul gem is the first not to fill the full bar, the soul gem above it is the size you need. I'm fairly certain this holds true with all enchanted weapons. There's no real need to quantify the amount a specific soul gives you.