I Just attempted my first Chain Swap, First I matched up the number of links on the old Chain (identical except for Color) and removed a few links on the new.
The Old Chain had the rear axle lined up just behind the second to front Notch (of 7) on the Swing arm and once i had the new chain connected, It was about a touch behind the first front notch on the 'Chain side' with the perfect amount of sag/movement. however on the other side (rhs) i couldn't get the axle to move forward that far, I was only able to move slightly in front of the second notch before something inhibited its movement (possibly part of the brake caliper mount)
So now i have the chain a little tighter than i would like 20mm ish flex ...should be around 25 to 35 mm, and also the axle is a tiny bit Skew (its a bit of a bugger really ).
Currently running a 44 tooth I could drop that down to a 43 and get some free play that way.
Good Idea ..ill check that now (although, it would seem strange if the markers are wrong on the swing arm) also I'm replacing Chain due to having a tight spot in some links on my last chain,(very long and wet Trip and not lubed enough due to fellow riders having none and using it all on their bikes) Both Sprockets are fine.