s7-1200 HSC and pulse input from a flowmeter

I am working with a 1214 siemens plc and I have connected the pulse output of a flowmeter into the digital input of the cpu with the HSC enabled for that input. I am able to read the flow rate and i have converted this value into the amount of pounds that are flowing through the meter. I have a manifold, which is about 4 feet long and with a 1 inch diameter, and i have 5 valves tied into the manifold at a 45 degree angle. The manifold then makes a 90 degree turn and flows into the flow meter. I have a valve 3 inches after the flow meter. I am having issues in calculating the offset that is involved. For example, if i input of a set point of 20 pounds of chemical one, the plc will output a signal to chemical 1 solenoid valve and simultaneously open the end product valve at the end of the flow meter. I noticed that if i input 20 pounds, the end product which i weigh with a scale i get an amount more than 20 pounds. Is there an equation or a way in order to calculate and account for the amount i need to subtract from the setpoint in order that the product that flow out is equal to the setpoint? If you need anymore information please let me know. Thank you

Just a thought
Most flow meters / sensors require a straight pipe both before and after the sensor to minimize the turbulence in the flow. That can give you bad readings
Check the manual it should tell you what it is.

It seems like the easiest thing to do would be run it at 20lbs a few times and measure the output. the difference between the two would be your offset.

Sounds easier than volume calculations to me. Unless you're running multiple fluids with different density.