A solution to my problem definitely must exist somewhere online, but I definitely don't know the right terminology to find it. Essentially I am making a quick script to convert measuring units of oil (cst and wt) both of which are brand specific so are difficult to calculate the 'weight' of the oil. I have this basic information for the two oil units. Is it possible to find a formula to convert between these two units based on this information?
\begin{align*} 1000\mathrm{cst} &= 80\mathrm{wt} \\ 650\mathrm{cst} &= 50\mathrm{wt} \\ 200\mathrm{cst} &= 20\mathrm{wt} \\ 100\mathrm{cst} &= 10\mathrm{wt} \\ \end{align*}
I don't think it is possible to know exactly how the amounts in cst units are converted to the amounts in wt units. Either the relationship is non-linear or there is some rounding going on.
If the relationship is based on $1\,\mathrm{cst} = k\,\mathrm{wt}$ but the result of the calculation is then rounded to the nearest multiple of $10,$ there are various values of $k$ that might fit the given results. An obvious choice would be $k = 0.08$, which is exactly for the first conversion and also works out for the other three conversions with rounding to the nearest multiple of $10.$ But a factor $k = 0.082$ would also work.
I don't think there is any way to reconcile the given figures with a linear factor with the result rounded to the nearest multiple of $5.$ If the rule for conversion had that form, the first equation says $k$ must be less than $0.0825$ (otherwise the linear conversion would be from $1000$ to at least $82.5$ which then would round to at least $85$), but the third equation says $k$ must be at least $0.0875$, since otherwise the linear conversion would give less than $17.5$, which would round to $15$ or less.
Conversion in the other direction would represent only a guess as to what the weight in cst is, since rounding causes multiple possible cst weights to be represented by the same wt weight.