How to predict and adjust the spice level of dal?
Firstly, it has to be said - the way you learn to judge quantities is … practise.
However, there are several factors at play here…

&quot;Indians&quot; [sorry, I'm generalising a whole continent into one word for convenience] don't eat mouth-stingingly-hot food at every meal.
Some food is mild, some is 'go-for-it'. Some is rich, some is bland, some has heat, some has aromatics.
There is a North/South divide on 'spiciness' or 'heat' with the Southern continent going for hotter examples, Northern more dependant on aromatics.

Western &quot;Indian&quot; restaurants have tainted our view of what &quot;Indian&quot; food is.

Dal can be pretty bland anyway.

Your recipe is towards the bland end of the spectrum of possible dal recipes. The 'Sri Lankan-ness' in it is really only the coconut &amp; perhaps the mustard seed.


So, that done with, in a broad sweeping generalisation kind of way, to your issue.
The recipe as written is missing a lot of what I would consider a 'basic' dal.
For a whistle-stop basic chana dal…
Boil dal with turmeric for half an hour
Fry cumin, garlic, onions
Add tomato, ginger, green chilli
Add chilli powder, turmeric, garam masala [or supermarket 'curry powder' depending on availability], asafoetida, coriander powder, dried methi
Add to dal mixture
Simmer until happy.
If you want to trick this up to feel more Southern or Sri Lankan, substitute coconut milk for some of the lentil boiling liquid, sub coconut oil for the ghee, add curry leaves &amp; black mustard seed. Chillies, fresh green or ground red to taste.
Some of the ingredients you can add more later. Chilli powder, garam masala or 'curry powder' if you want a quick boost.
Other aromatics such as cardamom &amp; clove, if you cheat &amp; use them in powdered form, you can boost at any point. They're not as potent as whole, but they're a quick fix.
Another cheat is garlic powder, which is used by many restaurants rather than fresh - again, not the same as fresh, but can be used as a pep-up at any time [watch it doesn't go lumpy, make a thick slurry with water first to be safe.]