How to chalkline in a metal roof

I am building a lean-to on a shed. I extended the roof and was able to get some matching corrigated tin for the roof. The original shed is not exactly square, so the new roof is not exactly square either. Rather than try to cut each sheet to length and compensate for the slight crookedness of the roof, I just installed the sheets whole, and left the ends hang over the edge of the roof. Now I want to cut that entire edge, and want it 3 inches beyond the facia board. The plan is to run a chalk line. The problem is that I dont have anyone available to hold the other end of the line. Since there is no wood under this end of the tin, just the tin itself. how can I hook the starter end of the chalkline? Yes, I did think of putting a sheetmetal screw in the end, but is there something easier, and that screw would leave part of the drilled hole in my cut line.
Thanks
*31*
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Clamp... Visegrips... Tape...
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Snip a short cut, trap the line in the cut.
--
______________________________
Keep the whole world singing . . . .
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.