Assuming your tank is 48"L x 20"W and you need 4" depth, you will need about 200 pounds of SD. If you want to use wet live sand on top of the SD to prevent SD from floating to the top and cloud the whole tank for 2 weeks, then use about 160 pounds of SD and 40lbs of LS. This is just an approximate guesses.
You can use www.purearagonite.com , they have a sand bed calculator that will help you determin how much sand you need.
Well I can tell you this, I put 200 lbs of southdown in a 75 gallon tank and it gave me a bed which went anywhere from round 8 inches in high spots to 5 inches in lows. 100lbs and then some coarser substrate on top should be more than enough.