Generally within 10%, and it will depend on what you are flying over - broken canopy gives variable readings of 50' or more which is your 10%. I really only need them when less than 1000' agl, and specially below 200'. Good for reminders about wheel extension and critical decision points.
They don't look forward at all, so they have no purpose as a forward-looking radar to warn of approaching terrain. They just look straight down.