In my opinion, the first step is to find out how much nutrients the plants in your tank 'consumes' during the course of a single day. You can use test kits (which may not be all that accurate) but I prefer to look at things such as pearling rates etc which you will have a feel for with experience. Generally, fast growing plants, grown in high light and high temperature will consume more nutrients.

You will want to add nutrients to the point where adding more does not result in better growth/metabolism. In other words, you don't want the nutrients to be the limiting factor. However, you will also need to ask: is this what I want? To speed up growth so I have to prune/maintain/rescape more often?

Once you figure out how much to add in a day, you need to look whether dumping the whole amount in at one go will stress the inhabitants or not. I now add about 10 ppm Nitrates, 2 ppm Phosphates and 0.1 ppm Fe + traces each day but feel it might be too much in a single dose so I do it in the morning and once at night. If you don't have fish or invertebrates then it should not matter that much. The photoperiod begins around noon.

If you only add smaller amounts of nutrients each day, I don't think it makes much difference when it is done since the fluctuations won't be that large anyway.

If your Fe is not chelated or if the bond is easily broken, you want to make sure to dose it separate from the phosphates. I've not had any problems with Fe EDTA dosed with phosphates.