Should I decrease the amount of yeast I add depending on how much sweet I add (I use jaggery) while making sweet bread?
I understand that sugar (and by extension jaggery) helps yeast multiply at a very fast pace, faster than the rate of multiplication in flour alone.
So is there a rule or ratio by which I must decrease the yeast for every unit of sugar added?
I understand that sugar (and by extension jaggery) helps yeast multiply at a very fast pace
This is only partly true. Yeast is a living organism and can only live under certain conditions, including a certain osmotic pressure.
If you start from pure bread dough, then adding a bit of sugar to the dough (or to the preferment) can make it rise faster. But adding sugar in quantities sufficient to make the bread taste sweet will slow down the yeast growth, as opposed to making it quicker. There is even an upper limit for how much sugar you can add per 100 g of flour, and it's not recommended to use recipes which prescribe more, as they turn out poorly.
So I wouldn't change the amount of yeast, unless you make the empirical observation that your yeast is overfed (recognizable by speed of raising and the changed smell).
No, there is no "conversion rate" because in baking there is no fixed ratio of yeast to flour or other carbohydrates from the start.
Simply put, the time your dough needs to rise is a function of yeast's growth rate over time - if you start with little yeast, you need more time until you reach the same result.
The growth rate can be influenced by ambient temperature, available food (sugar or flour) and inhibiting factors like salt.
You might be tempted to add sugar or more yeast to "speed things up", but frankly, you probably don't want to. Too much yeast will result in a distinct "yeasty" taste in your finished bread, too fast a rise results in uneven bread structure and "flat" taste. With yeast, slow and steady is usually the better choice.
So yes, you may absolutely decrease the amount of yeast (at least if you start with a typical recipe from the Internet or similar sources), especially if you add sugar. I usually at least halve the amount without batting an eye, but whatever you do:
Don't judge rising times by a looking at your watch, but by looking at the volume of your dough.
A good recipe should always define rising by volume, e.g. "until doubled in volume" or similar, not just give a fixed time.
