How do I know where to clamp it on the seat tube?

always a tricky one… i hate that bit…

But why any different than the normal 1p trick ?

Same as for any other front mech, ideally so that the cage passes about 1mm above the teeth of the big ring as it shifts up onto it, or as close to this as you can before it touches something else.

I thought using a 3x front mech on a 2x you had to mount it higher to get the shifting right I could be wrong

Give it a shot and leave the cable a little long in case you need to move it, worst case it doesn’t shift very well and you need to tweak it a bit.

How high or low will depend on the frame though. On the steel frames I’ve had in the past the chainstay/seat-tube/downtube were always skinny enough for me to bring the mech low enough to have the 2mm/penny gap, but with the aluminium frame I’ve got now I can’t bring the clamp low enough before the cage hits the chainstay.
Moving the mech up results in pretty poor shifting tbh. That however is with a 32t middle as its what I had, with a 36t + I may have got better results.

I might be wrong, but I’m pretty sure you are using a 2×10 mech 😉

😳 if its the one that came today then yes yes i am cheers Tom

You would mount it as you do any other mech unless you have a full susser wings chain stay in the conventional place in which case it is worth taking the air out of your shock first to check the chain stay and tyre don’t could the front mech when full compressed.