With setting the padding values to Zero (0) – the padding-top is then:

/*** to quick css - better to place with the concerning page ID ***/

.mfp-iframe-holder {
  padding: 0px !important; 
}

.mfp-iframe-holder .mfp-content {
  height: 90vh !important;  /*** the wanted height ***/
  width: 50.625vh;  /*** the calculated width is then ***/
}


.mfp-iframe-scaler {
  padding-top: 178%;  /*** 100 x 16 / 9 = 177.88 ***/
}