It is said that B&W chromogenic films such as the Kodak and Ilford versions in fact produce better negs at EI 200. There is less grain at lower speeds in chromogenic B&W films. So one stop less would make sense anyway unless you really need to use full box speed.

If you have some( as in several films) I'd be tempted to take each scene twice at EI 200 then 100. Check the negs on return from the lab and have the "better speed" negs developed. On your second and subsequent films you will then know the best speed

pentaxuser