How can I determine how powerful certain herbs and spices are? Is there a list? I am trying to determine the relative strength or potency of flavour that comes from different spices and herbs. I often find myself lost when trying to blend recipes since certain spices can be quite potent and it is easy to ruin a recipe with a powerful spice from another recipe. The best solution is to divide the weight of the spices by the total weight of the recipe in order to apply a similar ratio in my own recipe, but this is extremely cumbersome.

A simple way to address this issue would be to add spices based on their relative potency. For example: if I have 10g of dried thyme and I want to add nutmeg to the recipe, and I know that nutmeg is roughly 10x more potent (just a guess!), then I can add 1g of nutmeg and I can be assured I haven't ruined the recipe.

I know spices vary in their flavourfulness based on their age, different parts of the world, how they're processed, etc., but there's also no doubt that some are far more potent than others. I know the relative potency of certain herbs and spices offhand, but there are many others I am ignorant about.

Is there a list or resource that ranks the potency of herbs or spices? Some numerical indices would be fantastic, but even just categorization (e.g., very strong, strong, moderate, weak, very weak) would be helpful.
I doubt you could even do this for different brands of hot chilli powder (for example). Never mind trying to come up with an equivalent between things that aren't equivalent. Fresh chillies are even more variable. Strong flavours like herbs and spices interact with the other flavours in a dish, so something you find equivalent in one dish won't be right in another. Example: cumin can overpower a mild curry if you're not careful, but a large batch of my slow cooked chilli could handle a huge amount, while too much cinnamon in either wouldn't taste right, and the curry could take a decent bit of ginger while the chilli would cease to taste like chilli. 

Ginger, mustard and chilli are all hot spices, but the difference is one of kind, not just degree. You even feel the heat in different places (chilli more in the throat, mustard in the nose). Further, one person's perception of these different kinds of heat will be very different to another's - I could say that a sample of mustard is hotter than a sample of chilli sauce. You could say the opposite, and we could both be right.

The solution, to a large extent, is to test when making an unfamiliar dish or using unfamiliar ingredients. Accepting some variability is also necessary. Most herbs and many spices can be adjusted towards the end of cooking, sometimes by using a different form. Of course you need a rough idea of how much to start with, but you won't get that by replacing some amount of thyme with basil.