@straychev get your usual size. dont size up or down. if you are a strict W30 and maybe a W29 in washes that run larger, then go with W30. measurements can be misleading (DC i dont mean that you cant take proper measurements!!! :P ) its just from personal experience.
im a strict W31 in 95% of thavars (5% W30 since newer washes are very thin or stretch a lot), shioners etc,
W30 in krooleys (and braddoms even though i dont buy any)
W30-31 in darrons-tepphars depending...