You can determine how much slug weight required in order to achieve desired length of dry pipe with certain slug volume that you will use. Please follow these steps of calculation below;
Step 1 Determine Length of slug in drill pipe in ft:
Length of slug in drill pipe in ft = slug volume in bbl ÷ drill pipe capacity in bbl/ft
Step 2 Determine hydrostatic pressure required to give desired dry pipe drill pipe:
Hydrostatic Pressure in psi = mud weight in ppg x 0.052 x desired length of dry pipe
Step 3 Determine slug weight needed in ppg:
Slug weight in ppg = (Hydrostatic Prssure (from step 2) ÷ 0.052 ÷ Length of slug in ft (step1)) + mud weight, ppg, in hole
Example: Determine slug weight required for the following data:
Desired length of dry pipe = 200 ft
Mud weight in hole = 12.0 ppg
Drill pipe capacity = 0.016 bbl/ft
Volume of slug = 20 bbl
Step 1 Determine Length of slug inside drill pipe in ft:
Slug length = 20 bbl ÷ 0.016
Slug length = 1250 ft
Step 2 Determine hydrostatic pressure required to give desired dry pipe drill pipe
Hydrostatic Prssure in psi = 12.0 x 0.052 x 200
Hydrostatic Prssure in psi = 124.8 psi
Step 3 Determine slug weight needed in ppg:
Slug weight in ppg = (124.8 ÷ 0.052 ÷ 1250) + 12.0
Slug weight in ppg = 13.92 ppg
Ref: 
Formulas and Calculations for Drilling, Production and Workover, Second Edition
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