Ah Roff... there's your problem bud. I was assuming you were doing Bimini/Fig 8.
Line to big leader - the rationale is that you want a join that will be as strong as your weakest point (ie your mainline). If the knot made is as strong as the mainline, you can't get better than that right?
With 0.35 (20-25lb) line... if you make a straight fig 8 to the leader, you will have (at best) an 85% strength join. ie your knot will break at about 16-17lb if it was done perfectly.
In comes the Bimini... which makes a double line of your mainline... with the double line of the mainline, you now do a Fig 8 to the leader - that knot made is now stronger than your mainline strength because it is doubled. If anything is going to break it will be your mainline, not the Knot! The other great thing about the Bimini is the amount of "give" or stretch it allows... it's amazingly forgiving...
So sorry boet, if you want to do it properly, just learn the Bimini.... you will never turn back.