Find the angle of the stairs so you can set the saw to that angle, get two off cuts and set them to the maximum space to regulations, which is 99mm.
Then measure the distance between the spindles, but along the base rail. in this instance it's 128mm.
Dry fit a number of spindles at the bottom of the stairs next to the bottom newel, in this case 6 spindles, then measure the remaining distance. You must divide that measurement by the amount of spacings not the amount of spindles. (6 SPINDLES YOU DIVIDE BY 7).
As long as the division is less than the 128mm then you are within regulations and you have the exact spacer length.