Determination of cooling water flow rate

You need to achieve turbulence flow for each flow lines. Turbulence means Reynold number is over 4000.

First you need to know nominal diameter of your cooling channels.
Then You need water temperature and with the water temperature you find the kinematic viscosity. Below is viscosity to temperatures table. Pick yours.

Temperature (F) Viscosity (ft2/s) x 10-5
32 1,927
40 1,664
50 1,407
60 1,21
70 1,052
80 0,926
90 0,823
100 0,738
120 0,607
140 0,511
160 0,439
180 0,383
200 0,339
212 0,317

Formula is; R= (Flow Rate x 3160)/(Line Diameter x Kinematic Visc.) (F,inches)
Pull Flow Rate from this formula using over 4000 R value.

Then you have minimum flow rate requirement (gpm) per line.
Multiply this value with paralel cooling line number and now you have total flow rate requirement.
Buy a digital flowmeter and during mold validation, measure all lines flows and be sure that all lines are turbulence flow and record it.

Remember, different pressure drops with parallel hooking causes flow imbalances which means some of your channels may have 10000 R number while some of them are below 4000. You should make balancing with making bridge. I am not an expert but this is how i am working.

Also remember if you are using mono ethylene glycol for antifreeze add %10 - 20 more power to pump. you can also theoretically calculate pressure drops during plugs nipples pipes 90 degrees. But that wouldnt be a simple answer :)