Creating datum points for self leveling screed on bitumen covered concrete.

Hi, I am putting self leveling screed onto a bitumen DPM on top of a rough concrete floor. I need to lay it out of level to match existing doorways, Has anyone got any ideas for gauging the thickness of the screed apart from screws (which will puncture the DPM) or tripods? I was thinking of gluing on small shims of vinyl tile. Thanks in advance for any suggestions.

 

Itools level pins

 

Thanks Paul the perfect tool for the job. M