How accurate is PicoGraphics.measure_text(text, scale)?

I am using an Inky Frame 4 (i.e. DISPLAY_INKY_FRAME_4) and PicoGraphics.measure_text seems rather enthusiastic, for example when using the bitmap8 font this text at scale=3, “Ann Summers boss Jacqueline Gold dies aged 62” (it’s a BBC Headline from today), it is returning 648, however it easily fits on the display without wrapping.
Am I supposed to be putting a value in spacing? (I couldn’t work out what value to use so I omitted it, I did try a value of 1 but it seemed to have little effect)