Would google EI dosing for your size tank.
Nothing meant to be exact about EI, but rather a way to ensure no deficinecy.
If low tech ,and no CO2,excel,glut,etc, then I would use daily amount suggested for high tech and your size tank, but only dose once a week.
If high tech ,higher lighting,,then dose full EI amount's.
From what I understand, most algae is cause from co2 deficiency as opposed to other nutrient deficiencies. I would dial in your co2 till your fish start to notice, then over time adjust your fert dosing as you observe your plants. Like what others have mentioned, EI dosing is creating an over abundance to ensure no deficiency, I would just stick to what you have (40 gallon tank most likely doesn't have 40 gallons of water in it anyway) and so how your plants respond once your co2 is dialed in.
No way to know for sure. You need to identify the problem and then solve it. The only real way to do this is logically pick variables to tweak and then work on them one at a time keeping in mind it takes a few weeks for your tank to show a difference. Usually for low tech dosing 1x per week is sufficient. I would test your nitrate levels to confirm you are dosing too much ferts. A little extra is good, but if you have too much you provide ideal conditions for algae. If you are overdosing then cut back to 1x per week on the excel and the ferts. I would use the 40-60 gallon dose.