Moving game pieces using atan2().
I check to see if the position (px,py)
has reached the target (tx,ty) by
absolute values of positions minus targets.
If they havent reached the destination I increment
by the dx I got from the atan2() calculation.

This works as it is. But I cannot actually control the speed.
If I want it to go faster, say speed 8, there are certain
situations where condition if abs(px-tx)<3 and abs(py-ty)<3 will never
be met and piece will just go forever. And I cant fine tune the <3 either
because sometimes the math just wont allow it.

How do I write code or think about this to move my sprite so I can actually control
the speed and/or accuracy?
Must be able to move any direction at any angle.
Thank you.

speed=5	
px,py=getxy(location)
tx,ty=getxy(destination)
local angle=atan2(ty-py,tx-px)
dx=sin(angle)*speed
dy=cos(angle)*speed  --PROBLEM HERE

function _update_move()
 if abs(px-tx)<3 and abs(py-ty)<3 then --AND PROBLEM HERE
  --ARRIVED AT LOCATION
 else
  px+=dx
  py+=dy
 end
end

compute how many frames it will take to reach the destination
(distance/speed)
and stop when frames left to travel is <=1
beware of overflows when computing the distance, and of divisions by 0 if you start at destination

thanks for the response @RealShadowCaster. will take me a little while to figure out how to implement.