Thanks for pointing that out SS. Now fixed. I should have written if the spicing needs to be varied. You are correct that it would need to work in reverse. However, there are 2 things to consider here.
Firstly the spice mix used in this gravy doesn't contain any chilli and only a trace amount of cinnamon. The other spice possibly subject to non linear scaling is Cumin. I used a linear reduction of 50% to make a half batch. Applying the X 1.5 rule in reverse here with the quantities involved would be negligible anyway. We are talking here about one instance of halving in a relatively lightly spiced base gravy when compared to a dish preparation involving more concentrated spicing and including chilli.. When applying a non linear factor to increased quantity, one instance of doubling is linear, so one instance of halving at this quantity is consistent in principle. The non linear approach to bulk cooking is used dealing with multiple increases in quantity in comparatively highly spiced actual dishes. Not base gravy.
Secondly, I made the statement about spice levels being scaled back, if necessary, in direct reference to Andy2295's claim that you can't simply halve everything to make a half sized batch. He says it doesn't work and the ingredients need to be proportioned, whatever that means. It could mean scaling up or down. I disagree. As I've said previously, I believe that a Base Gravy, by its low spice nature, by design, is able to be scaled directly in proportion. I have no problem reconciling this with the non linear scaling of spiced actual "dishes". They are 2 completely different operations.
There are no spice pixies. It isn't alchemy or witchcraft. It's all explainable as simply how spices work. How or why is another question. ;D
Edit:
SS, I've just realized that I actually did (accidently and almost completely) increase the spices to the inverse of X 1.5 anyway without and thought or intention of doing so. You see out here 1 TBSP is 20 ml volume and over there it is only 15 ml. Andy2295's recipe for the spice mix is given in mainly 2 TBSP measurements so where he was stipulating 2 TBSP (30 ml by volume), I used 1 TBSP (20 ml), which is exactly the X 1.5 factor in reverse as the reciprocal of 1.5 is 0.66667. The Garam Masala and Cumin halved from 1 TBSP, I measured 2 level tsp. The cinnamon is linear though I'm afraid, but I'm sure the difference between 1/4 tsp and 1/3 tsp won't be critical.
The negative of this is that I haven't actually scaled the Base Gravy by exactly half as I thought I was doing.

Oh well. I still doubt that it will be making any real difference when you are using spice variations in the magnitude of 0.03 % by volume for the majority of spices, or less.