I've had 2 contacts today. In person with my Indian chef acquaintance (K) and Mr Ian Hemphill called me directly to discuss the point upon reading my email. It appears there is a little bit right from everybody. No scientific formula but you do not scale in a linear conversion.
Chef K said when asked about increasing 1 tsp of chilli per serve to a dish of 8 or 10 quantities.
No. You don't use 8 or 10 tsp. I was always told to reduce by 20% every time you increase servings.
I clarified to per double quantity. He replied, Yes.
I clarified to reduce by 20% after doubling spices. He said, Yes, that's right.
So Chef K's formula is not X 1.5 but X 1.6. Double it and reduce by 20% equals X 1.6.
When I asked him why he said: It is because the stove cannot burn or cook the heat out of the chilli as much. The cooking is different.
This is directly in line with SS's point about the change in cooking dynamics from bigger quantities. It is also a point raised by Mr H in our phone call.
My email was quite specific in relation to the linear scale or X 1.5 theory and even mentioned the X 1.25 method. During a lengthy chat he said that you do not scale chilli in a linear way, but rather a factor of between 1.5 and 1.8 depending on the type, quality and freshness of the chilli, the amount you are cooking and the size and shape of your cooking vessels. He added that it is also to do with surface area for evaporation, heat input from your particular stove, the style of dish you are making and the other spices and ingredients in the dish (some also being non-linear but probably by a different conversion factor). So scaling a dish is not just a matter of multiplying everything by an integer being the number of serves, or a random figure of 1.5, or the scientifically aesthetic square root of 2.
So this brings me to some sound advice provided here, which is to cook out your spices first in oil after having gone with approximate scaling at reduced quantities using a factor of X 1.5. Add in your tomato and salt etc, followed by pre-cooked meats or veggies and your base gravy. I would be using a thickened form of base gravy and my past experience showed me to reduce it's quantity per serve as well. Simmer. While this is simmering, and to finish the dish if necessary, final adjustments can be made using incremental, BIR style additions taking sauce from the main pot and putting the extra spiced result back in until you have the balance you need. The addition of a tarka or bunjarra at this stage could be beneficial.
This is good advice and the person who gave it can take credit if they wish because it turns out they were pretty much spot on in explaining a good bulk cooking process. Using this method it is best to be under spiced so addition can be made. Difficult to reduce over-spicing.
Note: You can't just throw extra spices in at the end, They needs to be cooked out and added.