FirinFlatTop, at least you're thinking about it. The problem is that you only have one data point and can neither interpolate nor extrapolate from one data point ... you need more information. There is definitely a place for interpolation (and to some extent extrapolation) in reloading, and reloaders have been using interpolation
safely for decades.
For example, you mentioned W231 and lead bullets. If you look at .45 Colt data on
www.hodgdon.com you'll find the following data for cast lead bullets and W231 powder:
160gr, 6.5gr to 9.0gr
250gr, 5.8gr to 7.1gr
If you use linear interpolation based on the data above (graphically or numerically), you will get the following load
estimates for the bullet weights shown below:
****************
ESTIMATED DATA
****************
180gr, 6.4gr to 8.6gr
200gr, 6.2gr to 8.2gr
215gr, 6.1gr to 7.9gr
230gr, 6.0gr to 7.5gr
****************
As is turns out, Hodgdon lists load data for those bullet weights anyway:
180gr, 6.0gr to 8.2gr
200gr, 5.9gr to 8.0gr
215gr, 5.9gr to 7.8gr
230gr, 5.8gr to 7.3gr
So you would have no problem using the estimated
starting loads since they're only marginally above Hodgdon's starting loads. But how about the estimated maximum loads? Well, consider load data for the .45 Colt, W231 and lead bullets from the Speer No.11 manual. Loads are listed for 200gr and 250gr bullets.
200gr, 7.3gr to 8.3gr
250gr, 7.1gr to 7.6gr
The 200gr and 250gr loads are 0.3gr and 0.5gr
over the respective max loads listed by Hodgdon. Graphically, it's clear that the max loads estimated from linear interpolation are more conservative than Sierra's load data (in this case). So what does all this mean? Basically it means that there is a place for linear interpolation in reloading and I would have no concerns at all using linear interpolation to find a load range for any lead bullet between 160gr and 250gr using W231, particularly if I were following good reloading practices i.e. starting at the min estimated load for any bullet of interest. I would even consider extrapolating for a bullet a few grains either side of 160gr or 250gr.