I'm still a little vague on what you're trying to do
but timedelta is for timebased things
like objects moving and animation that happens overtime
unless you want the y-coord to be changing, you should have that there at all
aside from that, I kinda don't know what you're trying to do
but it sounds like you might have a picking issue in there as well
but as an example
lerp(a,b,t)
gives you a number between a and b, determined by a 0 through 1 value of t
<img src="http://upload.wikimedia.org/wikipedia/commons/8/89/Bezier_1_big.gif">
lerp(2,4,0)
would be 2 because it's all the way to the first number
lerp(2,4,0.5)
would be 3 because it's .5 of the way to the second number
lerp(2,4,1)
would be 4 because it's all the way to the second number
also, if you just want something 16 pixels below where it already is
you can just do y = .y+16