You know how much volume will be increased due to adding Hematite into the system; however, you sometimes are limited to total volume due to limit pit volume on the rig so you need to calculate starting volume to achieve the predetermined final volume of desired mud weight. This formula below is used to determine the staring volume of mud with Hematite.
Starting volume in bbl = VF x (40 – W2) ÷ (40 – W1)
Where; W1 = current mud weigth in ppg
W2 = new mud weight in ppg
VF = final volume in bbl
Example: Determine the barrel of starting volume of 10.0 ppg (W1) mud required to achieve 100 bbl (VF) of 13.0 ppg (W2) mud with Hematite:
Starting volume in bbl = VF x (40 – W2) ÷ (40 – W1)
Starting volume in bbl = 100 x (40 -13.0) ÷ (40 – 10.0)
Starting volume = 90 bbl
In order to achieve 100 bbl of 13.0 ppg mud weight up with Hematite, you must have 90.0 bbl starting volume of 10.0 ppg mud.
Please find the Excel Sheet for calculating starting volume of original mud weight up with Hematite.
Ref book: 
Formulas and Calculations for Drilling, Production and Workover, Second Edition
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