Well, the general equation is:

E = L * l / e

where:
E = exerted force
L = load force
l = distance of load from fulcrum
e = distance of exertion force from fulcrum

I see no reason you can't assume that the weight of the lid is at its center of gravity. Let's assume that's at the center.

So far we have:

E = 30 lbs * 15.5" / e

Since you haven't yet provided specs for your springs, let's say:

E * e = 465 inch-pounds = 38.75 ft-lbs

(Note that we're talking torque.)

That all assumes that all of the forces are being applied perpendicular to the lever arm, which is not true. How you mount the arm is significant, and you probably want to mount it so that it applies the most perpendicular force when the lid is open, so that it won't pop the lid open on its own. And, obviously, the direction of gravity changes in relation to the plane of the lid as it opens.

I guess what you want is to find the normal force exerted on the lid by gravity when open and when closed and then find a spring that has a rating between the two.

I don't know how the springs are spec'd, though.