measure your nitrates in your tank right before you do your weekly water change. lets say they are at 10ppm.
ok. now figure out how much you dose during the week. lets say you dose 7ppm 3 times a week. that would give you a total of 21ppm per week that is dosed.
now test again at the end of the week right before your water change. if it is around 10 ppm
again then you know that you use a rough est of about 11ppm a week.
if you cut you're dosing by 50%you would be nearly bottoming out and i think would be a little risky. i would start by cutting back by about 25%. and watch carefully for signs of deficiency in your fast growing plants.
things arent gonna change overnight. cut your dosing and wait a couple weeks. keep testing your parameters. adjust accordingly.
remember that ei isnt an exact science. you're basicly just giving the plants more than they can use in a week so that you dont have to worry about bottoming out. you can adjust ei to whatever works best for you