I frequently receive questions from readers about how to change a recipe.  They may want to substitute ingredients, reduce calories, or make  the recipe "healthier" in one way or another.  The problem is, they send me the question without the recipe they are hoping to change and expect me to somehow be able to know the context.  Think of it like calling a restaurant you have heard about and asking, "How do I get to your place?"  You know that the first thing they are going to ask is, "Where are you now?"  They can't tell you how to get from here to there unless they know where here is to begin with.

For example, I recently got this question:

I have a cake recipe from Georges Perrier, Le Bec Fin, Philadelphia, that uses 1T baking powder and 1 tsp baking soda with 3/4 cup buttermilk and 1/2 cup good quality bourbon (Bourbon spice cake).  It's delicious as formulated.  I am substituting agave nectar (a low glycemic index liquid sweetener) for the sugar, which also requires reducing the liquid in the cake by 1/3.  If the target ratio is 1/2 tsp baking soda to 1 c buttermilk, and baking soda is four times as strong as baking powder, then presumably I need 2 cups of acid, not the 1.25 cups of the recipe??  I'm worried about reducing the liquid that much.  The other liquid is 3 eggs, easy enough.  Any suggestions?

Now, I'm not about to publish the recipe, or tell people that so and so eats vast amounts of Bourbon Spice Cake, any more than the new restaurant is going to take out billboards telling everyone your address, but it would sure help understand the context in which the question is asked.  It might even allow me to test my suggested fix in the kitchen to be sure I'm on the right track.

Without the recipe, the best I can do is assume some common reference point -- "Drive to the front of City Hall, and then go right six blocks to highway 16 ..."  Of course, those kinds of instructions may be way out of your way, and may not really help if you live in Podunk and I'm in East Eyebrow.

So, having no mile markers to tell me where the recipe is coming from, what can I say about the above question?

1. General recommendations are to replace the sugar with about 2/3 the volume of agave nectar.  Since agave nectar is supposedly sweeter than sugar, you need less of it.
2. Because agave nectar contains water, most substitutions require that you reduce the other liquids in the recipe.  Depending on what type of sugar is being replaced, the amount may vary between about 1/4 and 1/3 of the volume of agave nectar used. 
3. If you are replacing liquid sweeteners like honey or corn syrup, the ratios and the changes in amounts of other liquids may vary.
4. Substitution guidelines recommend reducing the oven temperature by 25°F (12°C) and increasing the cooking time slightly.

If your recipe, for example, calls for 1 1/2 cups of white sugar, then you need to use about 0.666 X 1.5 = 1 cups of agave nectar.  You also need to reduce the amount of other liquid in the recipe by 1/3 of 1 cup.  Now comes the problem, where to get that third of a cup of liquid.  If you take out 1/3 cup the bourbon, it's no longer going to be much of a Bourbon Spice Cake.  If you take out the 1/3 cup from the buttermilk, you may need to reduce the amount of baking soda by a proportional amount, just over half the total, but there may be other ingredients that are reacting with some of the baking soda, so you may not need to reduce it by that much.  If you reduce the number of eggs, then the structure of the cake may change.

There could be two other ways out of the conundrum about how to change the amount of liquid.  One depends on knowing the entire recipe.  Start by working out the ratio of flour to liquid in the recipe, remembering that butter is around 20% water by volume, each whole egg contains only about 3 tablespoons water (the rest is protein and fat), and that the buttermilk is between 1% to 2% fat.  Armed with this information, you have another option, which is to increase the amount of flour in proportion to the amount of excess liquid in the agave nectar.  A typical spice cake might have around the same weight of flour as liquids, so compensating for 1/3 cup too much water could just as easily be dealt with by increasing the amount of flour in the recipe by 5.3 ounces weight or around 1/2 cup.  Chances are that adding a half cup of flour wouldn't be right either, since that would change the ratios on spices, etc., but adding a 1/4 cup of flour and reducing the liquid by 2 tablespoons could very well work out, maybe without even needing to change any of the other ingredients.

Finally, you could cut the buttermilk by the required amount, in this example 1/3 cup, and then add buttermilk powder to replace it.  Since buttemilk powder is used in a 1 to 4 ratio with water, adding 4 teaspoons of buttermilk powder would compensate for the amount of real buttermilk taken out and should balance the recipe back up.  The water from the agave nectar will make up for the liquid lost in reducing the amount of real buttermilk used.

All of this assumes that the cake contains 1 1/2 cup of white sugar in the first place, which I don't know for sure. 

The point is a) messing around with the balances on ingredients can be fussy work and b) without having the whole recipe, it's hard to know how to change it.  The more information you give about where you are starting from, the easier it is to tell you the directions to get where you want to be.