Graham,
I used to do that with float-stop and a coffin-lead, but found 1. it's a pain to cast beyond an underarm/sidearm swing and 2. sometimes you want the backlead much closer to you to keep the line down across faster midstream current.
Now I use a clip-on (aka Captive) backlead which you just literally clip on to the line after casting and let it slide down into the water where you want it. Let it slide down in front of you, then by raising the rod-tip and line you can ease it out to where you want it. The line will straighten to the main lead as you do this. If you want some slack you can do this after.
I keep several different sizes available to suit the different rates of current I'm likely to fish in.
Once you're into a fish the act of lifting the rod/line will allow the lead to slide down and abut the main lead. Most of them have silicone sleeves enabling the lead to detach if the backlead gets snagged.
Makes things nice and easy.
After trying all sorts I've come to settle with Atomic Nutz Backleads as being the most relaible. Guess what they're shaped like?
