Wider gravel bars how much shorter stem

I’m considering changing from 420 bars with a 100mm stem to a 500mm bar but wouldn’t know how much to reduce the stem length by. I know bike fit is a personal thing but any suggestions would be appreciated. Thanks in advance

My latest gravel bike came with 400mm bars/100mm stem which felt fairly terrifying, swapped them for a 520mm PNW bar (flares out slightly to 605mm) and 70mm stem.

Big improvement – feels comfortable with more control/leverage without getting twitchy, just about at the limit of cables/hoses now though. Not sure I’d want to go much shorter than 70mm.

Shorter by 20, maybe 30mm sounds about right. 500mm drops will feel way wider than 420s but the actual difference in the triangle angles made by your arms to the grip area isn’t huge (or very precise considering how elbows bend). It’s as much about how a shorter position overall can work with the wider bar, if that’s how you like the bike set up. (520 and 60 or 70mm combo here)

I dropped from 100 to 80 when I went wider/flared but if you’re unsure, just buy a cheapish one (Planet X £15) in the length you think could be right.

Worse-case you’ve a spare.