To form the curry, I put the default ratios but suppose I wanted to think about making adjustment. Would it be better to do it in the beginning or when it has simmered for some time?
This is actually very difficult, because the best advice is to make notes and correct it the next time you make the dish.
The problem is that when making Indian curries, you typically bloom the spices in oil at the beginning. Trying to add spices later can result in a much different taste. You can sometimes get away with it with pre-made spice pastes if you’ve completely misjudged the amount of spice to use, but I wouldn’t suggest adding just any spice shortly before you’re going to serve the dish.
I don’t know if it’s traditional, but you could adjust flavors when serving by adding chutney.
Japanese curry, however often uses a curry roux, so the spices have already been cooked. You can get away with adding it much later in the cooking.
I know that there are also Thai, Malay, and Chinese curries, but I’m not familiar with cooking those, so can’t comment on when you can adjust the spices. (I think Thai curries mostly use fresh things like garlic, lemongrass, etc, so you can probably get away with adding more layer, but they might need to be cooked down)
There are definitely more in-depth and better answers, but it seems like you are looking for a more direct answer. I tend to like to add salt first when adjusting spices, and to let it cook with that for a bit, to see if it changes the flavor/my opinion on what could be added to make it better. I also tend to add any chile powder at this stage too, to let the heat simmer and settle through the dish.