I'm trying to get to the end of the Scutum Centaurus arm at the minute and the route plotter just stinks like an over-ripe cheese wrapped in a rugby player's worn sock wrapped in a dirty nappy out on the fringes. It won't even plot a route where it's drawn the mutter mutter spider web. If I can plot 100ly I'm doing well, it's like that all the way round the edges, just take it slow and you'll get there.
If you turn the lines on and then find the furthest towards BP they go and work back down them to where you are, trying to plot a route each time until it sticks. When you get there rinse and repeat.