If that’s 50mm you need a new tape measure.
https://www.diy.com/departments/diall-white-acrylic-round-edge-bath-seal-l-2500mm-w-17-4mm/5059340164533_BQ.prd?storeId=1336&&&&&gad_source=1&gclid=EAIaIQobChMIl7KO1t3ghQMVzIlQBh0MJwxhEAQYBiABEgLvzPD_BwE&gclsrc=aw.ds