Hi Doug,
I just went out and checked some dimesions for you. I have the same Thule
rain gutters as you and the low thule racks not the high riders.

It is important that the rain gutters get mounted high enough that the
inside of the rack towers clear the side of the pop top. Mine are mounted
so that the bottom edge of the rain gutter is 2" above the moulded
horizontal line on the pop top and the inside of the tower clears the pop
top by only about 1/4". The racks are quite stiff and I frequently carry a
170lbs laser sailboat on the rack with no problems.

I have the racks set fairly far back so the boat does not stick out the
front too far. Measuring back from the seam between the luggage rack and
the poptop the fwd most rack is 3ft aft and the back rack is 4 ft aft of
the fwd one.

Very important point, be very careful to drill with the canvas pulled
clear since the canvas is in the way and you risk drilling into is when
the rack is down. I recommend that you use stainless steel bolts and
oversize washers or a plate behind the rain gutters and also to use acorn
nuts to prevent wearing holes through the canvas.

By the way the only way to lift the top with a Laser on top is to do it
while standing on the luggage rack, I always had a piece of 2 by 4 with me
to jam the roof up because the thought of being crushed in the middle of
the night kept me awake.
Tony