IMO these 1/4" / 3/8" adapters are an abomination. Your valuable scope is then hanging on a couple of threads and it is no wonder we keep reading reports of damaged threads in scope feet. Good engineering practice would be 3/4" thread depth for a 3/8" thread but no scope, camera or tripod manufacture gets anywhere near this.
Best get a 3/8" adapter plate (Manfrotto part no. 200PL-38).