Hi Andrew, Ross hear. Your Ferret looks fantastic! Are they 101 tires you have on there?, looks well!
The white line is just so you can see the 'wind up'. You can line them up with the oil filler if you want but its a lot of work (undoing the prop shafts). If it were me i would just give it a drive on some wet grass, mud, or gravel, then mark it up. Any time the two lines dont match up after that, then a quick spin on the gravel will do the trick.
Regards Ross