I have a soil mix here i got off a video if it is any use to you.
Potting Soil 60%
perlite 15%
Building Sand 10% (Not beach sand as it contains salt)
Worm Castings 10%
Pre mix 5% containing N -12 P-6 K-6
N=Nitrogen
P=Phosporous
K=Potasium
The shop should understand the N.P.K value
Get this and mix it and theres your soil,personally tho i got a grow shop to deliver me a bag ready mixed