you follow the instructions - observe an dadjust.
so yes, if it says 4mL per L then you need 1,5 x 4 for 1,5 L
now, are they instructing a sane dose? that depens on what you observe. ratio of nutes AND concentration are equally important.
now, i don't have info on the overall concentration in EC or PPM. so, how frequently you fertilize depends on the concentration you are giving. higher the concentration, the less often you fertilize. if it's around 1,3 EC, you are probably fertilizing every irrigation.
i think biobizz is just some charged coco coir? so it's essentially soilless by the 8th week. just make sure you get 10% runoff religiously. stop choosing teh volume to give... you give enough volume of water to get 10% runoff. if consistent as to when you add that water (weight of pot), it will be the same volume each time so you can accurately mix up only the fertilizer that is needed with less carryover or waste.
you don't pick the volume. the characteristics of your substrate and size of pot dictate how much water is needed. it is rarely a neat volume like 1L or 1.5L