Yesterday I was doing some kitesurfing and my 15m speed was overflying the window.

It was steady 15-18 mph wind, the kite was at zenith, and the strap was let all the way out.

Seems like maybe i need to tune it differently than Flysurfer videos indicate.

Does anyone know what changes need to be made to a freshly tuned speed to keep it sitting a bit back in the window?

TIA

Was your kite collapsing, or just flying really far forward? They sit pretty high in the window but remain pretty stable. They aren't like arcs where you can just let them go, even in steady wind you usually have to make adjustments. Having said that... do a thorough mixer test and make sure your bar/lines are all even. Also, if you're letting the bar out quickly the kite will jump to the edge and want to overfly - have to be aware of that and pull in before it overflies. That's also the reason you want to let the bar out on upswing when you're moving. It all becomes automatic after a while.
Sorry if you already know all this - I have no idea how long you've had your speeds. I had the exact same 15 + 21 dlx, love those kites, but since replaced with speed 5s.

From the master himself:

it was odd... it was behaving mostly fine, but would just overfly and then get twisted up...

i did a mixer reset a couple months back, but only flew it once (no issues) after.. maybe it stretched out a bit... it wouldnt necessarily be odd behavior, except for the fact that it was fully sheeted out, so typically the kite would sit back in the window a bit (or even stall)...

i found this in my search for info, which i also found very helpful:
https://vimeo.com/9488886

also - ^the video jeffnyc posted above is the one i used to tune the kite

Yeah, that vimeo link is what helped me understand how the bridles function. He pops up on youtube still and is always very helpful.
If your kite sits around without use, things start to shrink. And when you fly your kite, the steering lines get less tension than the power lines, so your powers stay stretched, but the steering shrink (hence the extra knots on the leaders). Good to do the mixer test every few months. Eventually if you see wear, you should replace the yellow lines and pulleys on the mixer - you should have replacements that came with the kite.
Fully sheeted out your kite will be more forward, though it shouldn't be overflying (unless the wind was weird), mixer test usually corrects it. Sheet in or steer it over before it overflies.