place block in accurate position

i rlly dont understand how the nbt selector at @s if entity @s[y=44,dy=-0.1] really works
anyways i need to detect if certain entity that moves -0.11 y cordinates per tick has reached an y level, and if this is true it will setblock in the position the current entity is (is a block display is for block physics) anyways idk why but with the command above it places in the correct possition but before it reaches that position (in the second the bottom corner of the block display touches the top part of the  block in y=45 in sets the block so it is a small teleport there