Macros.....I'm trying to figure it out.

A percentage would be a percentage of the total calories, it wouldn't change your calories.

For example, if you are aiming for 25% protein. 25% of 1200 is 300, 300 / 4 = 75g of protein.
If you were eating 2000 cals. 25% of 2000 is 500, 500 / 4 = 125g of protein.

For what it's worth, calories determine weight loss or gain, and are more important for weight goals than macros. Macros affect satiety (which is highly individual) and can be important for health and fitness goals. The most common advice I've seen is a minimum of 0.6-0.8g protein per lb of your goal body weight, and 0.3g minimum fat per lb.

I just followed the MFP defaults, eventually increasing protein a little, and did fine.

Also for what it's worth, 1200 is the absolute minimum of calories, and you most likely don't need to be that low to lose weight at a good pace in line with your total goal.

You wouldn't have to change the number of calories when you change percentages. You would have to change the number of calories if you increase grams in one macro without a corresponding decrease in grams of other macros.

A gram of protein has 4 calories. A gram of carbohydrates has 4 calories. A gram of fat has 9 calories. A gram of alcohol has 7 calories.