The RMS/100 (x) vs Rat-Shilling (y) points (with x < 1700) are within 0.2 Rat-Shillings of the following graph:

$$y = 812.5-\frac{125}{1+e^{-0.02\left(x-630\right)}}$$

This was plotted using Desmos: <a rel="nofollow" class="external free" href="https://www.desmos.com/calculator/ttbqmk3f4e">https://www.desmos.com/calculator/ttbqmk3f4e</a>.

It is very strange that the centre of this logistic curve is not at 625 but at 630.