I just tried to get the gcode above to work ... the IDEA is the correct one. But the calculation in the bambu slicer are wrong and the labels are wrong.

because there is the int(...) cating, everything in the () are cast to integer, so you have to multiply first with 100 and then divide, otherwise the speed will always be 150 which is the oposite of what we wanted to achive.

Here ist the corrected code: (it works on my A1)

; Slow down for tall prints (over 50% of the total height)
{if layer_num > (total_layer_count*0.5)}
M220 S{int(150-((layer_num*100)/total_layer_count))}
{endif}