if you work on your own 500plus ltr tank min imo (others say different) but some earn more than others, and they will agree with what i am saying.
I know what you are saying when going into it, I personally had a 650l tank fitted for 2 men(I wanted 800) i have run out once with 2 never on my own, but on my own i have done 500l hence why i say 500l plus.
better to have to much than not enough as it will cost you lot of lost money if you dont get it right.
You are more than welcome to call me my numbers on on the home page of my site i have spoken to loads of guys on here about this.
good luck m8 and stick with it, scary it is, but long term benifits are far better and is a good business step again IMO

Ian