Just to be pedantic about it, it's actually 22.5 Degrees (half of the 45 degrees).
This is always a pain in the arse to work out. For the sake of expediency I suggest that you make a wedge shaped template for this and simply put it beside the charging unit. If its abut 8" it will cover most charges, and you can use a ruler to extend it beyond that for faster troops. You want to be streamlining your gameplay with aids as much as possible as the turns tend to take a while to get through relative to later editions of Warhammer.