Hi Ed,
I just did this step in the last couple of weeks and I got to tell you that I either didn't understand the masking tape trick or I wasn't doing it correctly (both I'm sure), because it didn't work for me either. I tried three different types of tape.
So, after tearing off the tape and pealing back the protective plastic, I began clamping the plexi to the white powder-coated frame. To my amazement a fine, gray contact line appeared through the plexi. I almost couldn't believe it; dumb luck struck again! So, upon contact with the frame a visible line could be seen through the plexi and that was my center point for drilling.
There are points where a little more clamping pressure was needed in order for the plexi to make contact with the frame, so I carefully used a stronger clamp pressure. If needed, I used a fine sharpie to "connect" the tow points and draw a line through where my 2" mark was for the drilling point.
I have the upgraded SC 15 Low UV canopy, so I don't know if that's why I could see a faint, visible, gray line upon contact; but, it worked. I fear the day dumb luck leaves me!
Hope this works for you.
Ryan