Help building a simple line drain placement template?

Hello,

I have a project where I have to design the surface of a platform. I have a few fixed points, mainly the platform edge and the plate, so I would like to find where to place the drain in between the two plates:

   

I have two different templates that do similar, but slightly different things, and I want to somehow combine the two.

The first template is using a fictive CL that I use to find where the drain would be based on side slopes and it looks like this:

L_1 and L_2 are fixed horizontally and vertically, same as R_1 and R_2

Point 1 is controlled by Slope from L_1 and R_1

Points 0 and 2 are controlled horizontally and vertically from point 0 (Horizontal distance 0,1m and vertical 0m). This is so the width of the drain is constant and also level.

I know that point 1 is not a really good constraint in terms of creating the side slopes of the drain, but it's the best I could do with the few points that I have, without creating recursive rules. 

With this template I get okay-ish side slopes for the drain, but it sometimes goes under the platform plate due to the height and width differences between L_2 and R_2, something like this:

I don't know how to set up rules to constraint points 0, 1 and 2 from going too close to L_1 and R_1 and ending up underneath the platform plate element. Here I would need to set up some variable slopes I guess, but that's another thing I need to learn how to do.

My other template looks like this:

Here the CL is null and is controlled by the actual horizontal and vertical alignment so the placement of the bottom of the drain is constant, points 0 and 2 again are constrained horizontally and vertically from point 1, but the side slopes are variable, only problem with this solution is, that I get slopes that are too steep or too flat for the drain:

I guess my question is, how do I make a template that has a variable (within range) slope for the drain, fixed distance from the platform plates and fixed width for the drain?