I don't think using an adapter will be less accurate, however, all torque wrenches have their range... if you use an adapter (either up, or down) it probably means you are operating further away from the mid-point torque for the design. I have a 1/2" Kamasa, which is the normal type with a micrometer-type handle, and a clarkepro 3/8" electronic which is great, as it beeps even when the screen is facing away from you. The 3/8 one is definitely not for you though, as it will only do about 100 ft lbs.
50th RR 1998 4.6L GEMS, 50th Defender 4.0L GEMS, Nanocom Evo (P38/ GEMS)
Formerly 200Tdi 90 CSW and SWB Series 3
SW London