It is known that in the small quantum parameter regime the Breit-Wheeler electron-positron pair creation cross section is extremely sensitive on the photon energy and on the background field. We present the dependence of positron yield on laser intensity in shaped laser under/over dense (gas/foil) plasma interactions with the help of analytical modeling and PIC simulations. This allows for establishing in situ laser intensity diagnostic by measuring the positron yield in experiments where multi PW lasers are used.