I'll just copy/paste what I wrote on a Reddit forum. I think for Xbox users the process can be confusing due to having no physical levers to calibrate. I hope this helps others:
Ok so I think I've kinda figured it out for Xbox users. Firstly, make sure the 'reverse on x axis' is set to 'no'. Then click on Begin Calibration. That button will then change its name to indicate what thrust you are setting. Now my mistake was to go messing around with the cockpit trust levers, setting them to the position they should be at for the thrust setting you're configuring, and then trying to save that as part of the calibration. But you need to do everything on the EFB, don't muck around with the cockpit levers at all! What you need to do is for each specific speed setting your currently calibrating is to click out of the EFB, point the cursor to a blank space (so that you are not going to click on any other interactive item by mistake) so that you can then use A & B to move the green bars up and down on the EFB (as if you were increasing and decreasing throttle as you would normally do) to the % you THINK that thrust you're configuring should be active at.
So when setting TOGA, for example, you would press A until the green bar goes all the way to the top, then click on the 'set TOGA thrust' button below. Then click on the button that originally said 'begin calibration' to move to the next thrust setting.
So there is some guesswork involved for Xbox controllers in choosing reasonable %'s because folks who have actual throttle controllers can easily just place them in the correct position. We just have A & B to press!
I used toga=100%, flx/tmp=85%,climb=77%,idle=0%
I hope that makes sense! The key bit was taking focus off the EFB to then use your thrust buttons to move the green bars up and down!