Why bother to create a SUPP WPT for a base leg? If you must have a magenta line on base leg why not define the point as a range and bearing from the threshold or other local navaid/waypoint in the scratchpad?
As BOAC says, fly the aeroplane: every day I see FOs carefully crafting complex imaginary downwind and base legs that they are never going to fly because that is not where they're vectored to. The need for a comforting magenta line at all times bespeaks an over reliance on the FMC imho - what more do you need than an extended centreline if on radar vectors to finals? The magenta line and the data it generates is misleading and inaccurate if you're not actually flying it.