I cut the below from a post I made before. Maybe it'll be helpful. Tom talks about there being a pretty wide range of safely overdosing, so as not to allow the possibly of their being a deficiency in any nutrient. Underdose, hurts plants and gives algae an advantage, overdose in a wide range and you don't hurt plants or induce algae, so why shoot for the bare minimum dosage and risk underdosing some nutrient and therefore limiting the uptake of some of the others when you can provide a reasonable excess and have some peace of mind and healthy plants. The water changes can reset your nutrient levels every so often. This always keeps your overdosing well in the wide, safe range and you hardly ever have to worry about there not being enough of something, or there being too much of something. Aim for the chest, not the head. Bigger target, better chance of hitting it, and you get almost identical results.
Here's the cut:
Tom's Estimative Index dosing method is based on a standard. He took fast growing plants and placed them under high light and good CO2 so that nutrient levels could be the only limiting factor. He then dosed ferts, increasing the dosages at intervals until he finally hit a point where the plants could not consume it all. That is the standard - the maximum uptake that fast growing plants under excellent environmental conditions can maintain. So what we do is take this dosage and scale it up or down to match whatever water volume we are using. It is assumed that we cannot have aquariums with much faster growing plants, better light, or better CO2 than Tom used in his initial testing. So, we cannot underdose using this method (assuming we scale his dosages to our tanks properly). We can however, and it is assumed that we will, overdose. It is assumed that minorly overdosing these ferts will not promote algae growth. To keep the overdosing within acceptable limits, we do 50% water changes. Lets say you dose for a week, but your tank has only half the volume of fast growing plants his had, or yours has weaker lighting, or your CO2 is out of whack. Not a problem. You will be putting in twice the amount of nutrients than your plants are eating, but if you do a 50% water change at the end of the week (this is not a rule, just a general guideline for starting out, it could be every 2-3 weeks, depending on your situation) then you essentially "reset" your tank.
The whole point is to avoid using test kits and having to "worry" over your hobby, thus increasing the amount of pleasure you get out of keeping your aquarium. That's why it's called "estimative". Basic dosing involves micronutrients (traces) and macronutrients through Potassium Nitrate and Mono Potassium Phosphate. You can get it straight from the horse's mouth at http://www.barrreport.com/